You may need to try live if you were doing frozen, but honestly I think you are still ok. I have seen mine go off for longer than that, but obviously starting weight is a factor, as well as if this was a fat snake at the starting weight or a thin snake at the starting weight. Assuming a healthy snake at 863, and a big poo could indeed be the weight lost, look for other signs for concern (lethargy, URI signs, etc...) and if nothing appears, then 3 weeks is fine. Finally, check your temps and other ambient conditions. If low temp, raise and stabilize temps to help. I always say if you are truly concerned, call a vet.