For me, it wasn't until fairly recently - January maybe. Others can think differently of course, but for me our performances in Aug / Sep / Oct deserved more points than they got (and both pundits and independent objective analysis like xG etc agree with me). Nov & Dec were made more complicated by the injuries. I know people go on about Palace and Ipswich games with a relatively full squad, but any team can have a couple of bad games. So throughout Nov & Dec, I was concerned about the drop off in performances since Aug/Sep/Oct, but hoped that it was more about injuries than anything else. More recently - for most games in Jan & Feb, we've just gotten worse and worse, tactics that worked on and off previously now hardly ever seem to work, and it's very obviously time for Ange to go and his appointment to be chalked up as an error (some might say it was a brave choice, others may say it was stupid, but surely everyone now agrees it hasn't worked and isn't going to work).
So when we talk about 'sticking with a manager', for me that means giving him a reasonable period of time (which of course is different for everyone) to see if it works rather than just sacking him (say) as soon as he's lost 3 consecutive games (not least because if we do that, no one will want the job). With Ange, it got more complicated by the injuries - I honestly believe our drop off in form might well have seen him fired in December if it weren't for the injuries. No one has a magical criteria of when is the right time to sack a manager, and some fans / boards will always be more patient than others, so we're bound to all have different 'timescales' or 'sets of circumstances' to say sacking a manager is the right thing to do at that particular point in time. So whilst I agree that 'there's no inherent virtue in prolonging an error', at the same time you have to give it time to see it if might not be an error after all, but just a blip.