Whichever you prefer.
You determine how much your starters play in the Misc Options section in your gameplan.
So, you could have your starters play 3 quarters, and put your rookies at #1.
Or, you could have your starters play 1 quarter, and put your rookies as backups.
In both of those scenarios, your rookies will play more.
The only difference would be that while your #1s are playing, your backups may also get into the game a little due to fatigue of your #1 players.