I wouldn't say there really should be a "set time" when handling your snake. Take him or her out, handle for awhile, and use your better judgement on when to put him/her back. You'll probably be able to sence when your corn's "had enough", and eventually you'll tire of constantly keeping it from slithering away to the darkest corners it can find.
I'd say the only amount of time that would be "too long" is long enough for your attention span to run out, taking your mind off your snake, and risking an escape.