Most airlines budget for complete cabin/galley/equipment refurb at D Check/approx 6-7 years
However common sense/practicality says that sidewalls/seats/seat covers etc. wont last this long and have to be addressed every 18 months or so...
You remove the sidewalls every 18 months on HMV and it is easy/cheap to respray them and 'clearseal' them at this time making them look brand new...same with seats/seat covers...however unfortunately most airlines are dropping this day to day housekeeping from their budgets...hence tatty planes
Few if any of the leased ex Danair B737-3/400's are still with BA most are from a variety of Lessors and have been delivered long since those from Danair departed
PS. I spend my days making 20 year old TU154's and IL86's look 'nice' and it's amazing what a bit of TLC after every flight can do to maintain the cosmetic appearance of an interior and at virtually no cost...BA or any of the majors has no excuse!
