Website Logout very soon.

There can be many reason for this.
as a First Aid,
1. Just check if you set a very short Session Timeout.
2. Check for memory leak or by any other reason your application pool is recycling.
3. Check if an anti-virus is accessing your bin folder frequently.
4. If you have huge data in database and using Entity Framework or similar thing, you must check for performance. This also causes Session Timout because of excessive memory recycles the application pool.

If not solved, please provide some more detail about your application.