During the growing season, the soil of your Venus flytrap should be kept damp at all times. Depending on pot size and how hot and dry it is where you are located, this may require watering your Venus flytrap daily.
Venus Flytraps should be watered when the soil is just barely damp and should be watered to the point where the soil is thoroughly saturated then allowed to dry to the point of being just damp before watering again.
In the summer months, smaller pots can dry out in just a day if the weather is very hot and dry. However, most of the year, there is usually 2 to 3 days between watering even for the smallest pots and sometimes as long as a month goes by in the winter months between waterings.
When Dionaea are dormant, their soil should be kept just moist, not exceptionally wet. The rule of thumb is that when you touch your finger to the top of the soil, just a few specks of peat moss should stick to your finger and it shouldn’t feel wet. If growing in long-fiber sphagnum, it should feel damp to the touch, like a moistened sponge, but not so wet that it seeps water when pushed or squeezed.
As always, be sure to use water suitable for your Venus flytrap. Most growers use distilled water, reverse osmosis water, or rainwater. Distilled water and reverse osmosis water can be purchased from most grocery stores for around $1 per gallon.