12 miles round trip, with about 4,300' elevation gain (per Dan's Hiking Pages) seems right in line with experience. This agrees with miles on trail signs too. The return from the peak has a few stretches of uphill, which is why this site's Cucamonga Peak front page gain of 3,900' is too low.
Glad to put my share in kitty. Good that you've already done so much groundwork here. Thanks to your write ups, I'll be trying Big Butch Wash for my first time on Sunday.