You can do that with user defined settings. The user can set a time period and if the app does not detect any change in the data coming from the GPS or speed/cadence sensor, it can stop the ride through code. Having an app auto start and stop an activity for you isn't really practical but it can be done. Having an auto start/stop feature in a cycling app, in my opinion, will most likely not be ON very long once the user's rides get screwed up because the user didn't do certain things needed for the feature to work correctly. It's probably the opinion of the app developers, as well, and another reason they're aren't any apps with that feature.