Depends. Can take up to 4 days for some. Usually if it does take this long, your female drops the majority the first day, then another couple onm the fourth day.
Just wait til she is very skinny and very hyper (as if she is very hungry) then take her out (on most cases with guppies the gravid spot will be very light - although they can stay dark but not often)......
One of mine delivered 8 babies exactly a week ago today. She hadnt had anymore by the next morning and she didnt look happy being in the birthing box so I put her back into the main tank. She is still swimming around with a belly full of babies a week later and showing no sign of delivering the rest yet!