There should be a line chart on the reel or with the manual if you have it. 40 pound mono seems pretty heavy unless you're really fishing a bad area. I never go over 20 on a spinner, too much more and you can't cast shyte. The exception may be if you're ch#^^^^^&g bait.
Maybe 25 or 30 then. No need to go higher unless you're slinging huge lead or going to really be abusing your line. Why do you need to know how much? Just fill it...