Short haul about 6 days as DaveReidUK said.
Long haul (747/777) takes about 9 days.
Using an average of about 50 aircraft a year - BA has about 250 aircraft. Total time? 5 years!
Incidentally, some aircraft were painted in Shannon, Madrid, East Midlands and Dublin during the bmi takeover as there was such a rapid influx of aircraft that the paint shop simply had no space.
Champ