Ok, a bit of background. I started my Ngondro practice at July 27 of this year in the Dudjom Tersar lineage of Nyingma Tibetian Buddhism. I was given the first part of the practice, including taking refuge, prostrations, and a tonglen practice to do after prostrations.
When I was given the practice I was told that I should do at least three prostrations a day and 100 is very good. Usually after about 30 I end up stopping in a sweaty heap and a sore lower back, though I have been able to do prostrations every day except one since I obtained the practice.