It depends to an extent on how old the snake is. A healthy hatchling-yearling will shed around every 4-6 weeks, because they are growing so fast. As they reach adult size the frequency changes, so an adult may only shed 4 times a year.
If an adult has an injury to its skin, it will shed more often while it heals. Frequent soaking and sheeding in an adult can be a sign it has mites. For people breeding their corns, the first time a female sheds in the Spring is usually when she's going into breeding mode and going to be receptive to mating. When she's due to lay her eggs, she'll shed her skin 10-14 days beforehand. Then almost immediately after laying, she sheds again.