Happens to me several times a day.
I only get booted off of AnswerMug and have to log back on about once a month or once every two months. Other than that, I stay logged on 24/7.
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I see the 4 to 5 week thing happens with you also.
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One would want to think it was a "caring" issue... perhaps to see if we died since our last login.
Or a security thing like banks do. Although banks give you 2 minutes before they automatically log you out, not 4-5 weeks.
Maybe it's a courtesy to make sure we dont forget our password. 
Honey Dew 