When I needed to use suppositories, I used Dulcolax bisacodyl suppositories. Those will take you from light sensation to absolute urgency in about an hour tops.
It's much like needing to go to the toilet really badly (because it's an irritant it feels vaguely like you've been eating mild chili for a couple of days). You do eventually need to devote more and more of your concentration and strength to not filling your diapers. I never got to a point where it actually overrode all my concentration and strength - I tried holding one for as long as I could and I think ultimately it topped out about 50 minutes and stayed that intense until I voided.
However, the urgency does eventually get so bad that if you stop giving it 100% of your concentration, or 100% of your muscular strength, you void. I had multiple occasions where struggling with a jammed cubicle door, or speeding up my walk slightly, led to relatively involuntarily filling my diapers. With the time I mentioned above where I waited for the intensity of the suppository to top out, I'd decided that if I made it to one hour without filling my diapers, I would get out of my diapers and use the toilet. I thought the intensity of the sensation was fading, but just getting off my bed diverted enough effort from holding that I involuntarily filled my diapers pretty heavily (and ended up just saying 'screw it' and sleeping in them instead of dealing with a messy diaper right then and there).