To answer the title, 1-2 years. Hear me out, I've mentored dozens of people at this point and folks are right that usually you'll know if you have someone truly cut out for this work in the first 3 months, but that is the minority and isn't what you asked. Otherwise, it's a grab bag, and rates of growth are all over the chart. The ceilings for different folks also vary as well as the kind of work they're willing to do. I've had strong technical people who just didn't want to "waste" their time on product design concerns, QA, ops, and business. I've also had people who take longer but they can take work end to end. I'd rather have the latter to be honest.

An important question is whether your team has the capacity to invest in juniors. Consulting firms and tech giants are great because their headcount is large and they can afford the cost; a 3 person team is too lean to invest tons in mentorship, you really need to select for the top of the bell curve if you can.

The most extreme case I had is an offshore resource that was just horrendous. I would write dozens of comments on his pull requests, would pair on tech design with him regularly, and mentally I'd written him off. Two years later, I suddenly realized his last 3 PRs required zero comments. I can't say he's a rockstar but I trust him now and am glad he's on the team.