Get to know your particular snake over time. It will give you feedback in a subtle and sometimes not so subtle way about how much and what type of outside the enclosure time it can tolerate. Watch for any changes in feeding behaviours especially.
It can be even something like just not the right day for that snake to put up with you and you'll have to adjust to it (unless you are doing husbandry things, then the snake just has to deal with it for it's own good).
With our snakes (balls, boa and milk) we just try to be reasonable in our handling, allowing for the fact that these aren't herd or pack type creatures that really need much if any interaction with another of their kind or a human being.
~~Jo~~