I've been in charter for about 8 or 9 years at a "big PNW charter company". My C30 usually ran 12-15 weeks/yr. My C42 is less, running 9-10 weeks/yr.
There are a LOT of factors that influence that rate, so don't put that in stone! Most important is the number of similarly sized and priced boats in the same fleet AND other local fleets. You can get a good feel by poking through the inventory at ayc.com, bosuns.ca, and sanjuansailing.com.