A natural point for him to leave would have been 2014 after winning the FA Cup. I think all round it would have seemed right. As Ash points out, if any manager deserved the opportunity with the financial changes, it was Wenger. For him to choose to stay was not beyond the realms to lunacy. This, as it turns out, has not succeeded.
To be offered a contract by the board in the summer was sheer lunacy. For Wenger to sign was not. After all, he had failed, his reputation has taken a dent and it looks like he had no other Football options and he got a pay rise. To accept that was somewhat of a no brainer.