Crikey, it varies! At Manchester I've flown a base to final turn halfway down the runway (to land down by the fantail which dates the time I flew based aeroplanes from there).
At Barton, anything from half a mile out (only ever more than that if I was stuck behind someone flying a bomber circuit - not rare at Barton) to turning final over the fence (the Yak 52 liked that!).
At Liverpool in the Chipmunk I always kept the final very short so ATC had no problems fitting me in between the Airbusses etc (and I flew 120 knots clean to close-in base, then power off and slow down to VFE speeds lowering flap as those speeds were reached and turning final, then establishing fully stabilised at 60 knots on a short final).
There are other circumstances where different length finals apply.