It depends what you're doing/ how complex your game is. The Nvidia guy on the ue4 livestream was saying the worst situation is finishing a game and then trying to add DLSS at the end, and was stressing it should be done as early as possible in development.
Hmmm I guess that means adding it after shipping the game as a patch might present difficulties.
I guess the idea of simply ticking a box is a best case scenario.
Probably July/August at the earliest.