It would be very safe to acclimate the guppies over a period of a week for best of health. The stress from acclimating not only will take a physical toll on the body, but it will also decrease the immune system as it is being subjected to new bacteria, diseases, etc as you chance them over. While you could probably get them acclimated within 30 minutes, it's generally a good idea to let them acclimate as long as possible.