if I substitute a player 15min into a match, will that player still get maximum....

[Killerbee]

They get experience points relative to the amount of time they played (and thus gained experience/used condition). So you have to play them for a full match in order to gain the full experience points.
It's not a massive difference in gain compared to regular training though, so don't worry too much about it. Only the man of the match gains an amount that he wouldn't be able to gain with regular trainings.

[ThichDaBongFC]

Only the man of the match gains an amount that he wouldn't be able to gain with regular trainings.

when you hardly train your best player as his high quality, this free SP is very worthy