How long an Async Request keep alive in server?

I'm implement Comet in asp.net, and I'm using asynchronous post back to keep request alive in server. But I don't know how long can keep a request alive in server, Some guys said that need to end async request because server will alert 404 error if I keep the r开发者_如何转开发equest too long. How long can I keep async requests alive in server ? Is there limit time or un-limit time ?

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technet.microsoft.com/en-US/library/e99daa45-4a87-465b-86f5-820fea9d1117.aspx

Edit

Comet implementation (if you don't want do it it yourself)

https://github.com/nmosafi/aspcomet