Morning

I stumbled upon this 'fire door' which as you can see is 80% Unmarked GWPP and 20% timber.

Is there anything in writing which states a reasonable area of glazing within a door leaf?

Thank you

Looks like a timber-panel door and therefore most likely not traceable to evidence of fire resistance performance.  Your next 'go to' would be technical fire resistance performance data for the glass itself and most GWPP would most likely have been manufactured by Pilkington as the product 'Pyroshield' or its forerunner. Many would accept this type of fire door as 'Nominal or Notional'.

Where you cannot satisfy yourself of suitable product performance, a Fire Risk Assessor should be able to advise in terms of the suitability of a fire door with unknown ultimate fire performance given the location of the door and the fire strategy at the building.