By wet, we are referring to canned cat food or raw cat food. How long it should be left in your cat's bowl depends on the environmental temperature. Wet food will last longer in cooler temperatures. As a rule of thumb, I will leave wet cat food out for a maximum of 30 minutes. After which time any remaining food should be removed and discarded and the food bowl washed in warm, soapy water.
I have found that if wet cat food is left out for too long, especially in the summer months (in Australia), flies are attracted to it and warm, moist food is the ideal environment for bacterial growth. In ideal conditions, bacterial growth in food can double every 20 minutes. Which can make your cat very sick. If you find that you are routinely throwing out uneaten food, it may be a good idea to revise the amount of food you are feeding your cats.
Any food remaining in the can should be transferred to a glass or ceramic bowl, covered with cling wrap and stored in the refrigerator. Wet cat food can be stored for up to three days.
Many cats don't like to eat cold food straight from the refrigerator, it can be warmed up for 30 seconds in the microwave to take the chill off it.