You'll know it's time to clean if your dishwasher:
Begins to lose its effectiveness
Gets calcium-looking stains from hard water
Gives all your dishes an odor or an onion-y essence because you washed a glass bowl used to store a half-used raw onion
For times when the unit is just not cleaning like its usual self, it needs:
Baking soda treatment
Simply fill a coffee cup full of vinegar, or with baking soda, or one of each in the top rack and run on a normal cycle with no other dishes in the unit. You dishwasher will be its old self again.