The problem with Sulmet is that it is very hard on their systems. Sulmet does not work here. It's not effective against eimeria tenella, one of the nine types of cocci, which is probably what is in my soil, so I no longer use it. I always recommend using Corid as the first defense against cocci over Sulmet because if they already have intestinal bleeding, Sulmet has a tendency to make it continue longer, being so hard on the system. Treating with sulfa drugs or antibiotics when there is no reason to is an extra stress on the system, IMO, and only creates super-bugs that are resistant.
That said, with the issues here with cocci in our soil, every group of chicks that hits the ground (where the oocycsts are) gets cocci, so at the point I put them outside, I may begin a week long treatment with Corid (concentrated Amprolium) as a matter of course. If I had no cocci issues here, I wouldn't do that. 4-8 weeks is usually when you see the symptoms if they're going to get it.