I've never used a rapid rooter but in general I like my seedlings to get a bit pot-bound before transplanting so the root ball slides right out of the nursery 6-packs without breaking apart.
The 6-packs I'm using are of about a 60mL capacity, and the seedlings get 3-4 sets of adult leaves before wanting out.
At that time I also start them on nutes, beginning with a weak transplant-shock reducing formula that contains thiamine.
So the quick answer is, it isn;t a matter of time, it's a matter of root development.