It depends mostly on how much the repository is used. With one user checking in once a day and a branch/merge/etc operation once a week you probably don’t need to run it more than once a year.
With several dozen developers working on several dozen projects each checking in 2-3 times a day, you might want to run it nightly.
It won’t hurt to run it more frequently than needed, though.
What I’d do is run it now, then a week from now take a measurement of disk utilization, run it again, and measure disk utilization again. If it drops 5% in size, then run it once a week. If it drops more, then run it more frequently. If it drops less, then run it less frequently.