This is a pure maemo setting and has nothing to do with Timenow. Above the checkbox there is the time limit for the backlight, and if I remember correctly that also sets the time after your phone automatically locks.

If you are wanting an app that locks your phone every time you cover your proximity sensors, I dont think there is one. This kind of behavior was once in some version of Timenow because of some error in my system, and the phone was not really usable. The screen would go blank all the time during normal use.