I will say that carbon is cheap enough you should not be reusing it. But for specific things you can, only problem there is really know way of knowing how much more it can remove from the water.
Length of time leaving carbon in, really depends, in the case of flat worm exit, I would only leave the first batch of carbon in for about 6 hours, then replace with a second batch for 24.
For normal carbon usage, I normally put it in the system for about 1 week once a month, or if I put it into a canister filter, I put it in for about 2 days once a month.
Kim