Well.. I know i've used it for a couple of hours without problem.. but I do recall that pianobar had an issues if you play Pandora for something like 5-6 hours your session will time-out and you need to re-authenticate. I haven't played it that long yet to try and capture that error and re-authenticate you automatically...

If you run it from a terminal and manage to reproduce the problem, the output should have the XML returned from Pandora that could help.