I just sorta *know* when to stop.... and I too have sat there and done it for long periods of time in the same spot. I lose track of time, whether it's 15 minutes, or an hour.
And as far as "do I do it everyday", well right now I'm in a remission, but usually I'll do it for about 5 days straight and have a day or two of break. It's like some sort of weird cycle with me.
But when you feel like you should stop and your brain is telling you to keep going, just tell yourself "I've done enough damage today... this is enough" and force yourself to put the blade down. Then go and do something else and don't go back.