It wouldn't build up at all if you run a full bowl of water through the system daily. The flush water flow is too wimpy to fill the hoses, so waste and TP build up in them, the the vac tank, pump, and duckbills too. VF owners are responsible for at least 75% of toilet problems posted in net boating forums and at least 90% of the problems are the result of using too little water.
I've written a piece I call "VacuFlush 101" that explains how it works (an amazing number of owners think they know but don't) and how much water it really needs to remain trouble free. I'll be glad to send it to any of you who want it...I'll need your email address to do it 'cuz there's no way to attach anything to a private message (now known here as "conversations" (which makes no sense 'cuz what else are forum discussions if not conversations??"
--Peggie