Depends a lot on what you are trying to accomplish with it.
If you have a low humidity situation (an iowa winter) you may want to control humidity with it. In that case, many members run it for a minute or two every hour or two or three.
If you are just trying to water and your ambient humidity is fine (my situation) then you may want to mist for a long period of time 1 or 2x per day. Indoors, that is what I do- for about 20-25 minutes per session. Most chameleons prefer to drink this way given the opportunity- they take their time moving into the mist and usually drink right out of the air- that's why I do this, but it isn't strictly necessary for most species.
If you are using a drip to water and just want to raise humidity slightly a couple times a day, a short mist (a minute or so) in the morning and evening is a good way to go.
If you are trying to control temperature outdoors in the summer (which I also sometimes do depending on what species I'm keeping and ambient temps- sometimes our highs reach near 100 here) then you can run the mist more often for longer periods or even constantly during the heat of the day to provide cool for the lizards. even the area near the mist but not in it, or in the cage after a mist for a while, cools the situation...