I get them twice a year. My doctor has started using a two day prep with split dosing of the laxatives. I'll be on clear liquid for the two days before the procedure, and the day before I have to take Dulcolax at noon and stay drinking the first half gallon of Golytely at five. I'm usually done in the bathroom around 8, but then I have to get up eight hours before my procedure (which means midnight to 2 AM most times) to drink the other half gallon of Golytely. I'll be done again in about 3-4 hours, then try to sleep until I have to go check in at the hospital.
I wear a diaper when I start taking the Dulcolax and throughout the prep, and I wear one to the procedure just in case. There's often still some liquid stool in my colon that they have to suction out, but I'm under anesthesia and don't even know until later when I read the report. I'll usually wear diapers for a couple of days after the procedure because I can still have accidents - this last time they did a chromoendoscopy, and I leaked a bunch of blue dye mixed with mucus into my diaper later that day. Pretty gross, but that's what the diaper is there for.
If you're concerned, talk to your doctor's staff and let them know that you're worried about having an accident. They can plan accordingly.