You can get the default screen timeout set by the system using this:
android.provider.Settings.System.getInt(getContentResolver(),Settings.System.SCREEN_OFF_TIMEOUT,-1);
So you have the default timeout and the last time the user touched the device. With this you can approximate the time left for the screen to turn off.
Though i am not sure how you would handle user touches to the NotificationBar which would reset the timeout.