I'm probably the WORST when it comes to breaks. Gimme a bottle water, my wireless headset, and some music and I will work for 4-6 hrs straight(sometimes 8) before I look up and be like, "Hey- I need to take a break/stand up and stretch,"- especially during the coloring phase; I'm usually more attentive during the inking phase after about 2 or 3 panels.
Usually during my breaks, I'll stretch or stretch out on my bed so my bod can stop feeling stiff/sore, get some more water, and/or something to snack on. I try to keep my breaks to 30 minutes or an hour, then get back in there and power out the rest of the work.