Given all that electronic hardware and servos and so on implied by this approach, have you considered simply adding remote control? You don't have to use it until it comes time to recover your aircraft, but perhaps you'll find you want to. Alternatively, if you're dead set against that, instead of using a timer implement a "dead man's handle" in the form of a radio transmitter/receiver pair in which the receiver initiates the recovery sequence when it loses the signal from the transmitter. So it'll recover as soon as it gets out of range, or when you decide to turn the transmitter off.