由于每个打开的会话持有到内存中不应设置高于20分钟（特殊情况除外）。

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查看Session.Timeout在MSDN网站

注意他们也给个说法的最低

  

应该也不能设置低于4分钟，因为客户很少当时导致会话状态丢失内做出回应。

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希望这有助于...

In ASP.Net the default Session Timeout is set to 20 minutes. Why so? Is there any specific reason behind it?

Thanking you.

It should not be set higher than 20 minutes (except in special cases) because every open session is holding onto memory.

From Session.Timeout on MSDN site
Note they also give an explanation for the minimum

It should also not be set lower than 4 minutes because clients rarely respond within that time resulting in a loss of session state.

Hope this helps...