So stretching your hand beforehand and not while/after drawing enables you to work longer? I feel a little stupid since I typically do it after I work on something. .o.
Also, when do you usually take breaks? I tend to forget, which I think is why I seem to cramp up easier. :P
I do a lot of clean up/inking by hand so I stretch my hands a lot before I start; I saw a chart on tumblr a while back with a bunch of different hand stretches for artists, but I just try and pull/stretch out the muscles that do a lot of the heavier lifting, like the base of my thumb. I also draw really "hard" and have a tendency to squeeze the pen really tight if I'm trying to do precise lines, so I can only ink for so long before it starts to hurt my hand. Stretching before hand really makes a difference; I imagine it's the same as stretching before going for a run or something. I crack my knuckles a lot too, but I don't think you're supposed to do that...
For breaks I estimate I take one every 2 or 3 hours, but I usually set some goal and then take a break afterwards, so for example I might animate/ink/color and then finish an entire shot and take a 15-20 minute break afterwards. If I've been working for 4 or more hours straight, I'll usually stop and force myself to get up and go do something else for an hour (usually videogames or some netflix or something).
I think it varies from artist to artist and it's more about trying to prevent yourself from getting burnt out. Sometimes I'll have a couple crazy days of crunch time (because of work) and will force myself not to draw anything for the weekend or something. That ends up being hard for me though because I still end up feeling guilty for not working on my stuff.