Bowel preps take "way too long" to work on me, too. Not as long as you, but because of the C Diff, I recently had to have a few colonoscopies and, so far, regardless of product, what is supposed to start working withon an hour or two seens to take many hours.
I recall that the last go-round I started the chemicals at 11:00 a.m. the day before and it was all SLOWLY starting to work around midnight or so, and it kept on working in the wee smalls and then...because of the procedure I was having, I needed to take Lomotil/immodium an hour or two before the procedure...just as the bowel prep was all starting to work...which was way stupid.