Depends on few things. First not all the tourneys I play regular, daily basis, I really do not like to play on Saturdays, compensate it, usually I play more on Sundays.
However, generally in a regular day, I have 20-30 potential tourneys, what I think, I will register for. However it depends on the first 4-5, as I don't like to play more than 5 tourneys in the same time. If those are going well, I play less tourneys, in that case 8-10. If I relegate early, mid or just catching the money phase, then I register more, that case it can be 15-20. In a normal poker day, lets say, I play around 12 tourneys. It takes around 12-16 hours.
When I get tired, I change position, watch some movie what refreshes me, drink coffee, smoke cigarette, go out from the house for fresh air, sometimes, if I am okay with stacks on all tourneys, I sit out and take longer breaks for 10-15 minutes including the normal 5 min breaks.