I've been wondering about this as well since Justin Trudeau was elected Prime Minister here in Canada in 2015. He's pushing for the legalization of recreational marijuana across Canada by spring 2017. Plus the fact that so many states in the US are legalizing it shows that most people have no issue with marijuana anymore, at least in the West.
However, I feel the org will still view it as a substance that allows the "demons" to enter your mind, and I'm sure we know former Witnesses who used marijuana before and will say the same thing (my parents included). I've talked about this with other witnesses I know and none of them agree with it other than the medicinal use of marijuana, or at least they're afraid to publicly say they see nothing wrong with recreational use.
I'm expecting the org to still say its prohibited, akin to tobacco use due to the perceived damage it could do to the body, even though thousands of witnesses use prescription drugs that can easily kill them if they abused them and yet there is nothing wrong with using any kind of prescription drug according to the org (I don't recall ever reading or hearing anything against any kind of prescription drug, and I know quite a few Witnesses who went to other countries like Mexico to get treatment/drugs that you can't get in Canada). This is something that I've been talking about as well, what's wrong with marijuana when the prescription drugs that some witnesses are on can result in death (with very stringent instructions on how to prevent overdosing), or how alcohol can easily kill you if you drank a lot of it yet its still permitted to drink?
At best, they won't say anything about it. At worse, they will outright condemn recreational marijuana in an Watchtower study article, possibly in one of the fall or winter 2017 magazines. The middle ground is making it a conscience decision, akin to having to carry a firearm for your job, where if it is found out/you say you are using recreational marijuana then you will be denied "privileges".