I would only guess that the time limit was discussed when AVB was appointed - and the targets set to be able judge the progress.
For me, a player takes time to settle and most people are happy to give a player a season to see how they fare. A manager, unless they're happy with the squad, has to build the squad they want and to get the assembled players playing in whatever style - so I would give a couple of seasons - or should I say main transfer windows.That would take him to Christmas 2013 to see the effect of his ideas. AVB's appointment was oddly timed though, as players appear to have already been targeted/bought and arranged to be sold before he officially turned up. So it's difficult to judge if the purchases/sales in the last main window were his or not.
I would also guess that the project targets set would have been a trophy and at least Europe for this season, with the trophy main target being the Europa. I just can't see Levy being happy with potentially dropping down the league without somehow keeping our profile up in Europe.
As for SAF at Utd, didn't he have four or five dodgy seasons at Utd when he first took over?