It took DS a day or two (because it is pretty mild, there are no immediate explosions - but when it does happen, get a party pack of baby wipes ready, my friend!), but he didn't have a build-up like your DD. The important thing is to keep giving her the stuff even after she goes. We stopped occasionally and we were back to square one. The idea is that you use it to get their bowels moving again over a period of time - at least a month or two. You can use it longer if necessary without any long term implications, apparently. Obviously giving her lost to drink will help the process along.

ETA: All of the dosage instructions are in the box. I think they have a schedule outlined for impaction, which I am assuming she has (where you increase the dose each day that nothing happens). We only give DS a sachet a day. If you don't have the instructions let me know and I can call you and give you the info.