As a surname, Cannon only goes back to the early to mid Middle Ages, when surnames were in general being formed in English-speaking countries.
As a given fn, the very earliest Cannon would've probably been used would be about the 16th century, when it could've been a mother's maiden name transferred. However, it's more likely that Cannon would've started to be given in the Victorian Era, when surnames for boys was quite trendy (and the surname didn't have to have a familial connection, either).
But, most will see Cannon as a given name to be quite trendily 20th-21st century. Indeed, if you click here, you'll see just how modern the name is popularity-wise: http://snipurl.com/da22.
Miranda "Multiple exclamation marks are a sure sign of diseased mind" -- Terry Pratchett