How long is a piece of string.
A simple aeroplane that I know well and have good recency in, potentially under a minute, possibly whilst taxiing. I tend not to do things on the ground from memory, because I don't need to, unless it's open cockpit in which case a checklist is a nuisance.
Something fairly complex that I don't know well, haven't flown before, or hasn't flown before (I've done a few first flights of new or rebuilt aeroplanes), I'm perfectly capable of taking 20+ minutes. But, I certainly won't do it blocking a taxiway, I'll tuck the aeroplane well out of everybody's way.
Cranfield is a particularly difficult place, because it's laid out with narrow taxiways towards the thresholds. Combine that with a lot of student pilots, lots of them flying aeroplanes that struggle to do a 360 in the width of those narrow taxiways, and you're pretty much certain to have this daft situation. Much trouble could be solved if runups for 21 on 18 before taxiing along A, or on Apron 5 before taxiing for 03. But this really is criticism of local layout and procedures - if you don't set up a training airfield to allow for variable and often longish runups, then you're going to get problems.
Certainly however, checking avionics and warming an engine up are best done somewhere else (either before or after brakes-off depending upon how, and whether, I'm paying for the aeroplane!).
G