Depends on the training plan, of course. That said, about 10 pops per season are reasonable. Assuming you had the perfect 18yo PF trainee - all skills high respectables, all time great potential, and you are training him to be a C or the U21 and a PF for the NT, his development would look something like this:
end 18yo. Trained in IS, ID and RB
JS: 8 JR: 7
OD: 7 HN: 7
DR: 8 PA: 7
IS: 10 ID: 10
RB: 10 SB: 8
end 19yo, trained in IS, ID and RB
JS: 8 JR: 7
OD: 7 HN: 7
DR: 8 PA: 7
IS: 13 ID: 13
RB: 13 SB: 8
end 20yo, trained in IS, ID, RB and SB
JS: 9 JR: 7
OD: 7 HN: 7
DR: 9 PA: 7
IS: 15 ID: 15
RB: 15 SB: 10
end 21yo, trained in JS, DR
JS: 12 JR: 8
OD: 7 HN: 9
DR: 12 PA: 7
IS: 17 ID: 15
RB: 15 SB: 10
end 22yo, trained in OD, PA
JS: 12 JR: 8
OD: 9 HN: 10
DR: 12 PA: 10
IS: 17 ID: 16
RB: 15 SB: 10
This might be one possible path. While I haven't had such a trainee, I did have such pops until 21 for a trainee with a worse skillset, so this isn't utopian. Note that such a player will get expensive fast, costing more than 200k/week at 22yo already.