Kay, like everyone else, will be helped by some years in an NBA strength and conditioning program. It will help him big time in terms of getting into the lane, finishing at the rim, absorbing contact, etc. It will help him from getting worn down easily in games(not saying he does, just talking generally).
I'm not expecting him to be ready to be a rotation guy next year. But as long as he shows real improvement after a whole off-season,a lot of d-league time, and another training camp, I wont be with the crowd that will probably be trying to run him out of town.
I'd say if by the beginning of his fourth season, if he isn't ready to be a legitimate rotation contributor, then you can write him off. That's a rule I'd use for any player, really.
You most likely had a good idea after two full years, you most likely know for sure after three. If it doesn't happen at all at by the beginning of four...time to buy a plane ticket to Shanghai. A lot of guys do't even get the three years they should get. I think Kay is more likely to get more time than most, give Gilbert liked him and wrote a nice little check to get him. Maybe not three full years, but I'd be shocked if Kay didn't at least get the entire season next year and then at least a camp invite the following year.
I really hope he succeeds, though. He absolutely has a very exciting and useful skill-set, if he can put it together, I think he'll have a long productive career. And eventually he'd be backing up Kyrie, so it's not exactly like people expect you to be amazing/ Just serviceable.