A lot of clubs wait until the situation is critical i'm guessing out of gratitude for what they done previously but with Watford it's no messing about, as soon as the decline/stagnation starts their out and in my opinion that's the correct way, Brighton another club benefiting from making that tough call before it was too late (they were bankers for relegation had Hughton stayed).
I thought Gracia had been a good appointment, granted they had a questionable start this season but feel he deserved more time. I think he would have kept them up relatively easily as they have a decent squad.
Far far too early but this is what Watford do and they haven't sacked a manager for two years so basically 20 years compared to other clubs.
Quique I thought did a good job for them in 15/16 and was surprised when he was ditched end of that season considering Mazzari didn't adapt at all to premier league. Still his record since is unspectacular and tough start with Arsenal and Man. City and Wolves away their first three games.