How many calories are in an apple fritter donut?

The short answer is... it depends. There is considerable variety based on brand, size of fritter, etc. Some examples might help, though.

Starbucks Apple Fritter: An astonishing 790 Calories, including 37 g of fat

Dunkin' Donuts Apple Fritter: "Only" 290 Calories, with 13 g of fat

It seems that the "average" fritter is more in line with Dunkin' Donuts than with Starbucks. Having said that, if your fritter requires both hands to hold it because it's so big, or if it tastes *really* good... you might want to consider it closer to the Starbucks standard. ;o)