10 or 20 Watts?
Miss and you'll burn a hole in your scope.
I think you mean Milliwatts. 10 to 20 milliwatt is only going to be visible in the sky if it's a 532nm Green Pointer.
All pen pointers shouldn't be left on more than 100 seconds, then turned off for 10 seconds and back on for 100 seconds. It's a duty cycle.
It all depends on the batteries. If you can get your hands on some Energizer E2 Lithiums, they should last you an extremly long time, probably wont have to change the batteries for months.