How do you value rookies in a draft for a keeper league?

[reenum]

I've been reading some sources that say not to draft rookies early.

I'm in a draft league that allows 3 keepers per year. Keepers can remain on an owner's roster for 3 years. In this context, I'm trying to figure how to value hot rookies like Wieters and David Price.

Any tips on when to draft rookies in my sort of league?

[Yoda]

In a 3 keeper league with time limit, I wouldn't worry too much about rookies.

Our system allows each team to keep a lottery pick: must be a rook or drafted after the 20th round in the draft. In this format rookies are a bit more important.

[C2ThaLo88]

With 3 keepers, I wouldn't put too much value into drafting a rookie who will only help you for half the season (if that). My question though, is what if there are no restrictions on how long you can keep a player?