I am currently scoping a lifecycle project to replace some defective/none conforming fire doors. At the moment I'm up to 23 (59%) of the doors failing inspection, at what point should I just call it a day and replace all the of the doors in the building.  To make matters worse it is grade 2 listed.

Depends on the building, the defects, the extent of the listing (in G2 it's often the building façade only) etc as to whether the existing doors can be accepted, upgraded/repaired or replaced. 

Its sleeping accommodation, I was hoping there was a percentage of failures that would mean that the regulations would tell me to replace all doors instead of making an induvial case to the building owners for each door.  When you look after 1500 buildings with 45000 doors you need all the short cuts you get

Commercial or residential? Guidance for blocks of flats determining whether all new or upgraded/repaired doors are appropriate is here: https://www.bedsfire.gov.uk/Business-safety/Premises-fire-safety/LGA-fire-safety-in-purpose-built-blocks-of-flats.pdf