The General Entry Books (or registers) will detail how long your ancestor was held at a particular parole depot, and note when he was discharged, usually stating via which
The record you require are ADM103/594 Parole Prisoners at Okehampton and Oswestry 1809-1812. This is at the National Archives, Kew. If you find your ancestor in that list then it should state when he was released, and probably state via Plymouth as that was the nearest major port that was the centre of the POW system in Devon.
ADM103/611, 612, 613 and 614 contain alpahabetical lists of French parole prisoners that may also give you some information, but not all entries are very detailed in these registers.
Hope this helps.