In general, it is almost impossible to over water your Venus Fly Trap. In fact, Dionaea can survive being completely submerged in water for a couple of days. However, if Venus fly traps are kept too wet for too long, it will lead to problems. It’s best to water your Venus flytrap enough to saturate the soil and then water again when the soil becomes just barely damp.
This is especially true when your Venus Fly Trap is dormant. This is mostly due to the fact that during this time, it is more susceptible to mold and fungus. So, during dormancy, the soil should be kept just barely moist.
Also, remember to always use only distilled, rain, or reverse osmosis water unless you have verified that your tap water has a very low TDS (total dissolved soilds) measurement. You can read more about watering your Venus Fly Trap on the Venus Fly Trap watering page.