Once per season at the end of the season. About 70 hours a season. Less this year.
I would check my level each time I got gas. Over time, I saw no use so now I check less often.
When you drain the drive this will not drain the resevoir.
Since you remove both screw covers on the drive you will not have a vacuum to pull the oil from the reservoir.
Just a note of caution, not related to your post, but here goes.
If you use a pressure washer be vary careful on your drive.
Reduce the pressure, use the tip that gives you a less intense spray and keep some distance between the tip of the wand and your drive.
I found this out the hard way.