Joramco in AMM (who Boeing picked for the B787 retrofits) is pretty good with retrofits, once the first retrofit is done and certified (takes about two months) they can do a tip to tail retrofit of a widebody in 6 weeks. This is with the caveat that they don't find anything in the structures while taking out the old interiors. The retrofit of the LX A343 in 2018-20 is a perfect example:
A340 refurbishment
HB-JMH : 06DEC18 - 06MAR19
HB-JMI : 07MAR19 - 25APR19
HB-JMC : 26APR19 - 18JUN19
HB-JMB : 11DEC19 - 20JAN 20
HB-JMA : 30OCT19 - 11DEC19
My take is that they will have 1-2 widebodies in AMM each time. This means the have to reduce the schedule or get additional aircraft to cover that.
HAECO will do the 16 B777 retrofit in HKG: they are also pretty effecient once certified and trained on the first aircraft. Finnair did the A359/A330 retrofit there.
https://www.flyertalk.com/forum/finn...ry-2022-a.html
The plan for the refurn is from H2/2024 thru 2027.