It's not a great answer but taking breaks kind of depends. Personally we tend to put the brakes on when it's been a few hours and/or if hand is tired.
Usually we go and play a game or something, go on a walk, ect. Just doing Something Else in order to let those tasty art/writing energies recharge and body rest.
Also -- absolutely listen to your body!!! If your hand is hurting, do not continue, and let it rest. Maybe even for the rest of the day. We got some nasty nerve pain a few times from ignoring it and it is not hard to accidentally damage your wrist or other important bits by just going nonstop. Contrary to what some people think, art can very much cause hurts, even permanent things, if you don't find time to rest.