As often as you like and/or as quickly as possibly after each time it
finishes a run of CHKDSK.
Or are you asking "How often *should* you run this?"
Technically - you shouldn't have to run chkdsk unless you think you are
having hard disk trouble. However - I recommend running it before you
defragment. So you would run it as often as you run defragment on your
drives.