He'd get tired just not after a single hour. Even back in Z we had Goku and Gohan spar for over 24 hours straight. Freeza is too strong to need to put any real effort into fighting Zamasu so it'd take a very
very long time to get exhausted.
But to actually engage in the What-If...by the time Freeza realises he can't actually put down Zamasu, there would probably be too many clones for him to keep track of. His best bet would be using his vastly superior speed and power to spam
Imprisonment Ball then to slam dunk the trapped Zamasus right into the centre of a black hole, which would require Infinite Speed to escape.
Now would Freeza actually think to use Imprisonment Ball and then banish Zamasu into a black hole? Who knows. Given his form is partially based off black holes and he's a literal space tyrant he would be more than aware of the properties of black holes and he certainly has the speed and power to bully even an army of Zamasus into one so...it's a possibility.