Looking back at the logs, I see that the first 1,000 hours took about six years, as did the second. The third 1,000 hours took 12 and 1/2 years and the last thousand took nine years. Over the years, the aircraft has built time at an average rate of 119.1 hours/year. N631S has been with me since late September of 2004; since then we've flown 755 hours together.
It's interesting that the Vref aircraft valuation service provided by AOPA says that the fleet average total time for '77 Skylanes is 3960 hours; as of now N631S is almost perfectly average.