DrChrono has an auto-logout feature built into the system that will automatically log you out of your account when it is idle for 30 minutes, then prompt the user to stay logged in with a pop-up, which will give the user an additional 5 minutes, for a total of 35 minutes.
The timeouts above are based on recommendations from The Open Worldwide Application Security Project (OWASP):
All applications should implement an idle or inactivity timeout for sessions. This timeout defines the amount of time a session will remain active in case there is no activity by the user, closing and invalidating the session upon the defined idle period since the last HTTP request received by the web application for a given session ID.
The most appropriate timeout should be a balance between security (shorter timeout) and usability (longer timeout) and heavily depends on the sensitivity level of the data handled by the application. For example, a 60 minute log out time for a public forum can be acceptable, but such a long time would be too much in a home banking application (where a maximum timeout of 15 minutes is recommended).