it is usually about 28 days sometimes sooner .. and the end of there tummys get blak and they get pretty round.. if you think she is gonna have them soon then i would move her to another tank if you can .. or you might lose alot of you have some other fish in the same tank ..
here is a link to my momma guppy givieng birth.. i was lucky enough to take pics of the whole thing.. http://www.aquariacentral.com/forums/showthread.php?t=64185