Form Authentication Cookie Expiration issue

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ASP.NET

I have an issue in authentication cookie experiy date in IE and in other browsers except Firefox
The following is the web.config setting i am using
<forms name=".ASPXFORMSAUTH" timeout="30" defaultUrl="~/Login.aspx"/>
For the above setting it is working fine with firefox the user can login into the website, but in other browsers the user cann't login into the website and i get 401 error.
Then I changed the timeout in web.config as
<forms name=".ASPXFORMSAUTH" timeout="350" defaultUrl="~/Login.aspx"/>
Now the users are able to login into the website in all browsers.
But the problem is authentication cookie expiry time is only an hour in all browsers except Firefox.
can any one please help me on this.
thanks in advance.