Personally, I'd say Theon is too far gone to ever return to his former self. And even if that remarkably does happen, he won't be the arrogant, cocky Theon we saw strutting into Winterfell with the Ironborn at his side. His days are severely limited, and I actually wouldn't be surprised if the Season 6 Premiere results in his death. Reek's redemption came to a head as he led Sansa to an escape route from Ramsay and the Boltons, and that could be his defining act. If he safely manages to escort the Stark daughter into the arms of Brienne, his task is fulfilled and he can die after salvaging a scrap of dignity. Just don't expect Theon to be Theon any longer. He's gone.