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.