I watch the history channel pretty frequently and they have these shows that predict what would happen to the world if something dramatic happened... for example: if all of the untapped oil still in the earth instantly disappeared, or "life after people".  I'm really curious as to what would happen if weed were made legal all of a sudden. I'm not here to argue the pros & cons, but instead to predict what would happen.

Day One:  Celebration on the streets.

 People would be parading around honking their horns in rejoice and happiness - similar to fans celebrating a Stanely Cup or Super Bowl win.  Head shops across the country would be conducting patio sales, and "test drives" of their inventory, asking passers-by if they would be interested in trying out the newest bongs on the market. They may even give out free samples of their newest "product" in the hopes of gaining returning customers.  Distinctive smells will fill neighborhoods, and pedestrians will "catch a wiff" around every corner.   Of course not everyone likes weed, nor thinks it should become legal, so concerned mothers, MAD and other protesters will start to gather around political office buildings in protest. Demanding that the government stops putting drugs in the hands of children, or stop promoting the lifestyle.

After 7 days:  The face of marketing - changed forever.

Visine commercials would be used to exploit it's ability to clear blood-shot eyes associated with pot use.  This is already known amongst users, but visine would then be able to use this as a marketing campaign - recognizing the percentage of their customers who use this product for this purpose.

Local farmer's markets will explode with farmers trying to sell this new product to locals. They may sell the plants, seeds, clones, or finished product... or maybe all of the above, to those who are looking to start their own gardens.  The extent of this may be so great, that new types of markets catering to marijuana agriculture will be scheduled to give them their own category.

Grocery stores and convenient stores a like will start putting advertisement tags that say something along the lines of "Great for the munchies!" in their chips and dip department.  Along with "Butcher's Choice cuts" and "Freshly baked" stickers on our food, we'll be incited by tags reading "Stoner's choice" or "New and improved - marijuana infused!" advertisements.

Other products will come with a new flavor.  Like pot-flavor vs. menthol cigarettes. Hemp liquor, amongst others.

All types of companies will be trying to advertise to the marijuana consumers... since we all know there's very many of them.  This previously untapped market will blow up into a huge marketing campaign for products from cars to toothpaste. This will cause further protest by people who believe that the campaigns and products advertise to children.

After a month:  The craze continues.

Although there are already many marijuana users, the legalization will bring these people out of hiding. There will be a lot of talk about this for some time.  It still wouldn't seem normal to see people walking down the street smoking, or to walk into an established business to buy the product no matter your opinion on the subject.  It will further divide users and non users, and the amount of debate will heighten.  In addition, regulation laws will be blurred, or not in full effect at this point.  We would be "working out the kinks".

6 months: The economy.

During the course of 6 months of total legalization, legitimate jobs will be created and sales tax will start to roll into government hands.  Evaluations will be made to estimate it's impact on not only local and federal economies, but the crime rates.   Small time marijuana growers/dealers will either continue to conduct business illegally without a permit/license or will seek out licenses.  Drug dealers who deal with more than just marijuana will probably exclude pot from the list, and focus more on more profitable and still illegal drugs... but most ma and pa grow-ops will comply with the new laws, as their hopes of the substance will be realized.  Most people grow to make ends meet in hard times, this will be more achievable and morally acceptable than it was before legalization. Individual growers, will have a leg up at first, but as corporations specializing in marijuana grow, competitive pricing will push out some smaller marijuana businesses, providing consumers with "Brand Name" pot, "Discount" pot and everything in between, ranging in price accordingly.

1 year: Education, regulation, and training.

Electricians who work residential properties will need further training to learn how to safely install grow operations in homes, in order to prevent fires.

Hydro companies will come out with packages for residential properties that will be in between existing commercial, industrial, and residential packages.  Grow ops will require inspections and permits similar to those of existing electrical permits - but containing regulations for grow operations.

Horticulture training programs and educational courses will include marijuana growing into the curriculum.  There will be separate courses tending only to marijuana growing.  

Scientists and engineers will be paid by large corporations to discover more efficient and patentable inventions for pot and it's production.  Potting soil, watering systems, preservatives, and machines that will help make mass production on a previously unheard of scale, possible and efficient.

The government will further gather information on pot, it's users, and it's existence on the market.  They will regulate a "legal age" and revise/create laws to cater to the wants and needs of citizens, along with laws that will create new taxes.  Politicians will have the hard task of including this new law of legalization into their campaigns when running for office. They will need to get involved - users or not, and take a stand on the subject.

People will be provided with more information on the substance, made available by the government.  People will be taught to see this as less of the hard drug that many think it is, but the potentially beneficial substance with side-effects. Alcohol may then be realized as more harmful and morally wrong, opposed to the other way around which is arguably incorrect.

Years following:  The calm after the storm...

Eventually the legalization of pot will become old news. People will grow accustom to this substance as being a part of society like alcohol and cigarettes. People will stop feeling special for being allowed to smoke in public, just like the novelty of being "of drinking age" wears off after your first couple trips to the liquor store.   A legal age will be put in place.  Like alcohol, businesses that serve marijuana will need to obtain a license, same for growers.  Advertising for the substance will be limited - like it is for cigarettes and alcohol (for example, in beer commercials, you'll never see anyone actively drinking or sipping from their beverage).  

This my estimate... there's many more aspects that I haven't spoken enough about, but I think this is how it will pan out in an average community.  Of course pot will never become legal "all of a sudden" there will be more regulations put in place before, or along with it's legalization.  This is what I think would happen if it were to be "all of a sudden".