The main full loop takes around an hour and 10 minutes at a moderate pace, make sure when finishing the first loop you DO NOT go down into the parking lot and ride through, but instead take the fork UP to continue the loop.
If you want to get into the 2 hour rage take a right at the first (I think) fork by the bridge which will take you up to a street. Keep going up that street to the Nike Missile Silos. From there you have some more options of where to go. It's a lot more climbing and may be a bit confusing, I've only done it once recently and didn't know the best route.
I prefer the main loop as it is 99% single/doubletrack and much more fun. However, you can ride a ton of variations if you want to get 2-3 hours. Good idea to look at the trail map here:
China Camp Trail Map
Do you have any more information on the Gala Ride tomorrow? I was thinking about riding there tomorrow but maybe not if it is too busy.