I think the answer is pretty much "how long is a (pretty long) piece of string....
Once you get the infant failures out of the way, a Mini engine will run a long time if its not (too) abused. ... and will stoically absorb more than its share of abuse anyway...
Mine is coming up to 30 years or use (and a little abuse) and something short of 150K miles. Oil pressure hasn't really changed but compression is falling ..and blow by is increasing (badly). But it still runs (enough to pull a 3.1 diff to over 100mph..... sorry Officer you must have misread that
Over the years there's been a bit of depot maintenance. The gearbox needed a (professional) rebuild.... when the shortfalls in a home build eventually became too hard to ignore. The radiator (installed in the 80s ) has been recored recently and I suppose there's been a couple of clutches in that time..
Just get on and enjoy it....
Cheers, Ian