It depends on the diameter of the hole in the drop ring mould. Counterintuitively, a smaller hole needs much longer to slump than a larger one. I got my info and fusing schedules from an ebook:
http://fusedglassbooks.com/Scroll down to the "Creative Fused Glass Drop Out Vessels" one. I have three of these ebooks and can highly recommend them for their quality. They contain loads of useful information, as well as the instructions on how to make the projects shown on the front.
Hope that helps! =)
Kathy.