How often should air compressor (for toilets) come on?

Hello,
We have four Microphor microflush air toilets on our new (to us) Ocean 70 that run off a Pumptrol air compressor. My question is, how often should the compressor come on? I tightened up leaks under two of the toilets the other day but the thing still keeps coming on every 2 hours or thereabouts. This timeline seems to be independent of whether there is light or heavy use of the toilets. I guess I'm finding it hard to believe that one flush every two hours is making it come on :/
If anyone could tell me what is "normal" that'd put my mind at ease.
Thanks!
Louise