You all has already started to sell your own trainee since long ago, don't know how long it'll take me to reach that stage lol
it depends! (the evergreen answer!)
training monorole will push high trainees. Lisanti was NOT fully trained since the beginning, and started with 5JS and 6 OD: the first two seasons he was trained at 50% for team's priorities, knowing that his potential would have stopped him. Looking at Scalambrieri, who was trained at full speed starting from "all 7" skills, he reached "star" cap in about 3 seasons and some weeks, allstar just after 4, and perennial all star (his) last season at 23 years and a half.
The trick is to have a leading trainee (Scalambrieri), a secondary (Lisanti) and a third (Masters) that are "pipelined" with approximately one season of difference in skills. And they must be picked with a clear design in mind: the training program of leading trainee would be perfect to them!
For additional instance, this season is very tough to me, with more than a risk to go back in III

I decided to train birole, to simplify gameplanning, improve defense by purchasing player ready in OD (I don't have time to increase it this season), and don't hurt gameshape because of minutes (let's say, 48' in training spot + 37' in second match of the week, too high). So, all my trainees are expected to be trained in JS for SG/SF more or less all season long, that should at least make my investments safe (people spend for a 16 JS!), and I carefully looked at players with appropriate skills to get the maximum from this training program.