How long should I wait to use a power trained player after power train?

[cookizzz]

I have noticed that some players have immediate impact (rating 8 and above) on their competition debut with their new club without playing any friendlies, while others players have a rating of 7. Some of the players who had a 7 on their debut do improve after a few matches.

I agree. New players may or may not have immediate impact. So my tendency to use new players slowly is just a personal preference. I do not follow it strictly.

For the original question about training, I never let my player stop training. I just checked the training results of a few new players. I don't notice that their training progresses are particularly fast or slow on joining new team.