I think he'll keep it for as long as he wants it.
It may be that he decides the time is right at the end of the season to retire, though I feel like he really does want to go out on top.
If he delivers another trophy & ECL qualification this season the board would struggle to look at that and see grounds for sacking.
I agree he'll want to go out on top - I wouldn't be surprised if he hung around for a few more seasons to try get his hands on the champions league trophy.
When you look at how changes in management have impacted United, the demise of Liverpool, Chelsea's bizarre rise & fall under Mourinho, and City's desire to get Pep in even though Pellegrini has been doing OK I'm not certain I want to see Wenger go without a solid succession plan in place.
If Steve Bould isn't the man (I don't think he is) it would be great to see his successor come in as his assistant for the season, and then have him take over after he is familiar with the set up.