How long does it take for a maggot to go into the pupa stage?

Maggots do not have gestation periods - they are the juvenille form of flies and are neither capable of reproducing yet nor do they give live birth (flies lay eggs that hatch into maggots). However, if you are referring to how long does it take for a maggot to turn into a fly, this will depend on the species of fly you are asking about.