Re: How often can I handle?
Don't handle them until 2-3 days after a meal (once the snake has had a good 3-4 meals with no handling at all). Aside from that, you can hold them for a few minutes each day or maybe one or two longer handling sessions a week. Mine usually all get handled for 5-10 minutes twice a week. Occasionally my favorites will watch a TV show with me ;)
Re: How often can I handle?
Actually, if they are hissing, pick them up for 5 minutes then put them back. If you never pick them up when they hiss, they will learn that hissing = not being held. As for how long to hold them, it really depends on the snake. As long as the snake is eating well, it's tolerating the handling. That said, if it goes off feed, diminish the handling sessions a little bit. Also, I wouldn't have them out for more than about 30 minutes. Once you've done that 2-3times a week and they're still eating, you can handle them longer. Just be conscious of their eating patterns.
Re: How often can I handle?
I have never had an issue with having my ball python out for up to a few hours every day actually. If he starts to curl up and try to sleep ill put him back right away, but my ball python is extremely tolerant of people, and even seems to enjoy being out for an adventure. I have read on other threads on the site that if the snake begins to show signs of stress such as no longer eating, you should handle him less. I really dont think there is any set time for handling you snake, it just depends on your snakes tolerance.