Bbu takes an hour for me, new condensing boiler can take 20mins and that is following the mi, a lot of mi now state ' check co2 with analyser, clean out condense trap and check gas rat, put the cover back on and **** off, so 20mins tops. To ask how long a boiler should take is a very open question as the above shows, I usually leave an hour for every boiller service to cover myself but most especially the new condensing boilers can take 40mins tops including tea and getting your money