It varies, but with Tan's reign, we've chopped and changed far too much, usually because they're cheap options and have to work under Tan and the board, of which some transfers are imposed on managers by the club.
Since Tan took over, we've had the following managers:
Dave Jones (part) - Did superbly, but could never get us over the finish line
Malky Mackay - Got us to the prem, but had a falling out with Tan due to dodgy deals
Ole - Shite
Russell Slade - Did a job, despite having one hand tied behind his back, but was a cheap option
Paul Trollope - Cheap and shite
Neil Warnock - Cheap, but one that actually succeeded. Left under a bit if a cloud looking back
Neil Harris - Cheap. Started well, then turned shite
Mick McCarthy - Cheap and shite
Steve Morrison - Internal promotion, so cheap. Started ok, but struggled soon after. Bit of an arrogant prick as well.
Mark Hudson - Internal promotion, cheap, shite
Sabri Lamouchi - Cheap, but kept us up despite having very little to work with
Erol Bulut - Cheap (I believe). Did well first season, but piss poor start now and sheer arrogance meant he had to go
Excluding caretakers, that's 12 managers in 14 years or so.