The viva is not a test of endurance, a space where you have to simply work through everything until it is done.
You can take a break. You could be offered a break after an hour or two. You could need one and ask. You can ask for a comfort break, a medically-related break, a break to think or check something in your thesis. You can have a break to compose yourself.
There are many reasons to take a break in the viva and no wrong times to ask for one, if you need one.