I guess how often to change depends on how often they pee! if they pee every hour, don't you change every hour so there's no pee gassing off ammonia near their butt? (ie. regardless of nappy?)

obviously not so much overnight - if she is sleeping through, don't put her down until she does a wee, then she would probably be dry in the morning, not necessarily by the time you get to her - but by she should only pee just after she wakes. although sometimes they wake briefly during the night to pee.

my dd 4 months has had 8 dry nights so far - dry including waiting the 5 mins for me to come get her up! that only happens if she does a pee on teh potty before bed though.