One time it took me around 45 minutes, and one time it actually took 3 hours! (The 3 hours time was VERY painful!! And I was even eating just soft foods a few days before the liquid diet.) During the 3 hour wait, I called the GI on-call fellow, and they said the prep should kick in within 4 hours, usually within an hour.
26 years old. Diagnosed with Crohn's disease March 2008. Battled a C. diff infection from Summer 2009-Spring 2010. Have been dealing with a severe flare for the past several weeks.