I have read on this forum that a rough guess average is 1-2% a year across all domains including .com. So if you have 100 domains, expect to sell 1 or 2 year on average. As it's an average it's entirely possible to not sell any for a few years. Non .com domains must be way harder to sell. Looking at latest sales, most are .com, so the expect a very low sell through rate for non .coms.
It also depends on the type of domains, how good they are, etc. A $100,000 domain would likely take much longer to sell than a $100 domain.
What counts as a sale? Does buying a $10 domain and selling it at auction for $1 count as a sale? I guess technically it does, but it's not a good sale.
Domainers that sell regularly seem to have 100s or even 1000s of domains.