There are two way to set session timeout property in ASP.Net.
First Method : Edit the following in your web.config fle.
<configuration>
<system.web>
<sessionState mode="InProc" cookieless="true" timeout="60">
</sessionState>
</system.web>
</configuration>
Second Method : Go to global.asax file and write the code below:.
void Session_Start(object sender, EventArgs e)
{
Session.Timeout = 60;
}
Also please refer the suggessions @ https://www.codeproject.com/Questions/464873/Validation-of-viewstate-MAC-failed