in the bw book is says to make the transition at 4months. this is a 'guideline' and definately not written in stone - whatever it seems your lo needs. if her naps are going longer and run past the three hr mark w/o her being uncomfortable/unhappy then it is probably time. i waited until 4 months to make the transition although i was going 'by the book' a little too much and taking it too literally, but should have probably made the transition just after three months.