Oh really??? Ok 2 contradicting points now. I've done this myself a few times but sometimes the player B got more than 12 minutes (like 18 minutes) but sometimes he got less than 10 minutes...
the reserve role, usually only impact it when the backup or starter get in serious foul trouble(normally that means early 6 fouls with let them play) or injurys where he virtually promoted a spot. Sometimes he also get subbeb when you have a high blowout win.
The same goes when you assign someone double times, and yes the minutes ain't constant, and you would get different result even with the same starter/backup combinations - but as a rough estimate i would say 10-12 minutes should be normal for the backup.