Well, as a general rule I wouldn't say all snakes digest extremely slowly, all species of snakes have different metabolisms. As for what I have here, the carpet pythons have very fast metabolisms, and the emerald tree boas are on the other end of the spectrum. Temperature also pays a big part in the process, as will as age and they actual prey eaten.
I would say on average most snakes will defecate once for every meal they eat, which could take anywhere from a few days to a few weeks. Probably 5-7 days in most cases if the snake is healthy and kept in optimum conditions.
That being said, there are certain species that have different needs. Emeralds tree boas have a much slower digestive process, the typical rule of thumb is to make sure they defecate after every third meal. If not its best to encourage them to eliminate by either soaking or spraying them down. A few people have gone so far as to build "rain chambers" to help with this.
