This is a good rule of thumb when deciding if a trail is too wet and muddy to ride. Most of our trail networks in Orange County officially close during periods of inclement weather, but anything on National Forest land does not. In order to not damage our trails by riding them when they haven't had a chance to dry out, pay attention to whether or not your tires leave a noticeable track. If they do, it's time to turn back. If mud is caking your tires and sticking to other places on your bike, it's time to turn back.
Damage done to the trail bed when the dirt is saturated is difficult to repair once the soil dries. As responsible riders we need to exercise good judgement so we don't negatively impact our trails.
The good news is, depending on the trail and soil type, it typically only takes a couple of days of full sun to dry things out enough to ride!