Using the default ASP.NET MVC template, I cannot figure out how to increase the FormsAuthentication timeout. It seems to always use 30 minutes.
I have followed Scot
My issue was only occurring in my production environment at my web host.
I found this link and generated a machine key to put in the web.config. Once I did that, the timeout value took effect.
Be sure that you are setting this in the ~/Web.config file and not in the ~/Views/Web.config file. Also, 50 million minutes is approx. 100 years, which might be hitting some date-related overflow in the browser. Try using a more reasonable number like 2 - 3 years (1.5 million minutes).
I had the same problem and adding a custom MachineKey with a ValidationKey to the web.config solved the problem. It seems to affect shared hosts.
I used this site to generate a random machine key: http://aspnetresources.com/tools/machineKey