I'll stick to my guns and say never.
I'd say the fight for marijuana legalisation in both the US and UK is very much a lost cause. For every one stoner that wants to get high in peace, there are 10 angry, mothers/politicians/paranoid androids that vouch for political correctness above the undeniable truth that weed is relatively harmless.

At the very least, not in our lifetime. There would need to be some serious bend in society for it to be considered.