Olivier, our team have created time-outs on app services. It will be available in release 5.10 (non-beta)
It works analogous to the time-out that we have added to web services recently. If after a specified amount of time no answer has been received from the app service, you can control what happens next in the error handling in the microflow.
We hope this feature will help you!
This is not possible for App Services yet, however we are in the process of releasing 5.8 where we made this possible for Web Services. It wouldn't be a stretch to add this to App Services as well (though on the other hand we do want to keep App Services simple to configure) but perhaps you could file a feature request for this?
For now if you need this really quickly you could do it using a Java action and some knowledge of java coding, for example start at http://stackoverflow.com/questions/6460664/how-can-i-interrupt-method-execution-by-time