Off-flavors from settled yeast aren't a factor of time as much as they are yeast health. You can definitely buy a good amount of time if you pitched the right amount of healthy yeast to start, provided ample nutrients (O2, FAN, Ca, Zn, etc.), properly controlled the temperature of fermentation, and maintained controlled temp after fermentation is complete.
After a longer-than-normal primary (>2 weeks or so), I'm more concerned with the yeast slurry than the beer sitting on top.
I recently wrapped up an experiment that was supposed to gauge the repeat-ability of brewing the same beer with multiple generations of a sacch/brett mixed slurry.
Because the majority of the mix was Wyeast 3724, the first generation stalled and required about a month in primary at 75F.
Even though the 2nd and 3rd generations were ready for transfer much sooner, the damage was done. The 2nd gen beer is lovely but has the slightest hint of warmth from higher alcohols. The 3rd beer is a complete mess.