When you are ready
Babies can sleep
in cribs whenever their parents are ready to move them there.
Make sure your crib meets federal safety standards and that you do not use or put in bumpers, comforters, blankets, or stuffed animals. And always but your baby on their back to sleep! http://www.Cpsc.Gov/cpscpub/pubs/5030.Html lists the current safety tips for cribs.