At a stretch, two years. The year he was in charge and the following year when we were out of European competitions.
We've seen top clubs frequently hire unqualified managers: Kovac at Bayern, Setien at Barca, AVB at Chelsea, Lopetegui at Real, etc. None of these managers ever do any lasting damage, and the clubs usually bounce back quickly after sacking the manager.
The truly damaging decisions that were taken at United over the past decade - hemorrhaging money on crap signings without an overarching plan for the squad, handing out massive contracts like candy, failing to invest in supporting infrastructure, sidelining the club's scouts, jumping from manager to manager without a cohesive, consistent philosophy or style of play (resulting in a mish-mash squad that's not fit for purpose) - were all actions taken by Woodward and, by extension, the Glazers.