Recreational use of marijuana will be legal in 15 US States by 2020.
In this case, "legal" means that a majority of adult residents of the state will have some legal way to purchase or grow marijuana, without a medical prescription, and will have some way to consume it, without breaking the state's laws.
The laws can be limited in many ways while still allowing the claim to be judged true, including but not limited to: age restrictions, location restrictions for where marijuana can be bought or consumed, time of day restrictions, felon status restrictions, or requirements for purchase of a license (so long as a majority of adults are eligible for the license, without need for a medical prescription).
The US Federal Government's stance of marijuana will have no bearing on the claim. If 15 states pass laws legalizing marijuana in the manner described above, the claim will be judged TRUE even if the Federal Government makes it difficult or impossible to sell or consume marijuana.
If at any time before 2020, 15 states concurrently allow legal purchase (or growth) and consumption of marijuana, the claim may be judged TRUE early. The claim may be judged TRUE even if some states later re-enact laws against marijuana.
As of the writing of this claim (April 2014), one state, Colorado, currently allows legal purchase and consumption of marijuana in the manner described above. While Washington has laws in place for legal sale and consumption of marijuana, there are no legal dispensaries operating in the state (yet), and so there is currently NO legal way for the majority of adults to purchase marijuana without a medical prescription, so it does not qualify.