Http Session expiration

When accessing a specific website I observe a "jsessionid" on the client side.

This identifier is used to identify the session with the server.

When does this session expire? Only when the client is inactive for a timeout-period? Or also after a fixed time (,which might be longer, say 30min.)?

Is there anyway to find out the timeout-period from the client side?

Also, is there a way to check whether the session is active?

Unfortunately, I don't know much about server side development and I could not find a clear answer in the web.