When I used Dulcolax-type suppositories in the past that were oil-based, I sometimes inserted two at a time to speed things up. If I only use one, it usually takes about an hour to fully work.
After switching to Magic Bullet supps (same active ingredient, polyethylene glycol-based), though, I find that one is plenty. The burning starts almost instantaneously, and cramps usually set in about a minute or two later. The longest I've been able to hold it in was about half an hour. Usually, holding them for 15-20 minutes allows me to have a thorough movement. Nothing beats the soapsuds enema though. (I have chronic constipation, and few of my bm's come naturally...)
The dosage is actually the same as the usual Dulcolax supps, but all of it quickly dissolves into the rectal lining.