Big Soylent fan here, firstly, Soylent isn't designed as a 100% meal replacement, or at least it isn't approved as such.
That being said, the inventor claimed in an interview that he had gone for a month on pure Soylent, and there have been many people who make similar claims.
Stay hydrated, Soylent does make you poop, it's just delayed because of the high fiber. Trust me, try it for a few days straight, you're colon will get cleaned out lol.
Make sure you drink lots of water, that goes for any diet, (lots of people are mildly dehydrated without realizing it.)