Like others, almost every day except the day after feeding. And like calmolly says, start out with short sessions and increase duration over time. I've had my little guy for just a few months.
After about 15 minutes I can see him wanting to get away so I usually let him crawl around on the couch and floor for a while then put back in tank. Sometimes (like if I don't have any more time to watch him) he'll go directly back in enclosure.
The key is to observe the snake and let it teach you when it likes (or tolerates) being held, and when it's had enough and wants to be on its own.