I don't think it will ever be FULLY legal like tobacco, I think it'll be regulated like HARD alcohol, which means varying levels of control and taxation, state to state.
But, I'll take on your premise, and address a few issues brought up by others.
Given the huge swathes of land in the US, we would very quickly hit the balance of production quantities, with the cost to produce + tax + whatever will hit what the users are willing to pay, and then take no more land.
Because it won't be profitable to do it. Just like any other commodity.
Even with about 1/2 the country smoking, and smoking ALL THE TIME, in 1965, we didn't use all the farmland for tobacco.
http://www.infoplease.com/ipa/A0762370.html
Most of the pot smokers I know treat it like alcohol. And don't use it at work. Just because it is legal doesn't mean it is a good idea, and usually a fireable offense.
So, in actual land use, not a real big issue.
But then, move on to hemp. OH MY GOD YES.
Hemp has so many uses I can imagine 20-30% of the total US farmland converting to it in a few years. And they don't care about getting rid of males. They are going to factory produce as much as possible.
The outdoor cannabis growers are fucked.
Let's say it one more time.
The outdoor cannabis growers are fucked.
Before you argue with this please read a few dozen links from the following google query.
https://www.google.com/search?q=pollen+from+gm+crops+affecting+neighboring+fields
The indoor guys should already have decent filtration systems for the air coming in to keep out pests. They may have to upgrade them a bit for the new pollen.
But the outdoor guys? Nope. So this will bring in a 2 tier quality/pricing scheme. Seedy outdoor pot or no seeds indoor.
Note: Seedy weed is NOT bad weed. It is annoying weed. Get rid of the seeds and you lose (ehhh, pulling out of my ass) 2/3rds weight. But the remaining weed is still good, at least if it is fresh and the genetics were decent in the 1st place. The sun is better than ANY indoor light (excepting high UV burn finishing lights), so don't expect the quality to be poor just because the plants are big and seedy. It'll just be "regional", ie: Hey, I got some decent idaho pot over here. Hah, I got some kick-ass texas weed myself.
But the ability to produce it outdoors for far less than indoors will cause a corresponding price drop, and then it'll only be an issue of snob factor. Think 20 foot high plants, are far as the eye can see, harvested with a combine. Actual cost to produce will go to pennies on the ounce.
Certain isolated areas like the Hawaii will probably enforce a no hemp law to ensure their pristine cannabis crops aren't harmed.
Which then leads me to: Imports!
Imported weed will compete with local weed. The only reason most of the world is against cannabis is because of the stupid US laws and our treaties with them. The whole world will be growing the moment it is legal at the federal level.
Hell, other countries are PISSED at us. Think about this. We send in the DEA to support their armies to eradicate huge areas of pot. Many people DIE in the process. They hear a helicopter, and they know the DEA is about to rain hellfire over them.
And then, what happens? We have huge chunks of California growing vast amounts of weed, and it is RARELY stormtroopered by the feds. Sure, you see the occasional story hit the news, but it is rare, as compared to the amount OBVIOUSLY out there.
Competing with the Columbian weed. That the US sent soldiers to eradicate. Those Columbians gotta be pissed. Looks like a mob turf war.
So when neighboring states start allowing weed trade (like neighboring states do with liquor), what stops them from importing outside the US as well? The feds. so before we hit full repeal, we'll see some interesting legal battles in those areas.
Various areas of the world are better suited for certain strains. And those strains will get a premium (think thai-stick, if you are as old as me). So those areas will have to determine what best to plant, and also cooperate with their neighbors. If you put up a 7 mile barrier between your crop and your neighbor's, and there is a forest in between, you MAY be ok. Not sure though. It'll be an interesting dilemma for those who stay outside.
Did I miss anything?
