I don't sit in any one spot for more than 15 minutes, 20 at the max. I have lots of places to hunt though and if I were limited to places to call, then I might sit in one spot longer.
I don't use an Ecaller, BUT from what I've learned about them, some guys let them play continuos and some guys mute them every so often, and both ways seem to work.
As far as switching sounds from one spot to the next, many factors come into play. IF they isn't any calling pressure other than yourself, and IF you haven't educated any coyotes from one spot to the next, then no reason to change sounds. Something to concider, alot of us will use more than one sound on a stand. SO don't be shy about trying other sounds. Calling coyotes is kind of like fishing in the sense that sometimes a guy needs to change baits/sounds to have success.