Dude, if you think the answer “Whenever you feel ready” is some kind of cop-out, then you are misunderstanding how romantic relationships work. It’s never a good idea to say it if you don’t mean it!
The general consensus in my social group is that anytime before 3 months is probably too soon, and anytime after that is fine (depending on how much time you’ve spent together during that window). But again, I must stress: don’t say it if you don’t feel it.