No - there is no "formula" that I know of that enables you to calculate this. In any case, it's so dependent on fire load, ventilation etc it would be impracticable to do for most situations.
You haven't said why you would want to try and do this - I assume that you're trying to show some level of equivalent fire resistance, for building control or risk reduction purposes? The old BRE publication "Guidelines for the construction of fire resisting structural elements" (ISBN 0 85125 293 1) gives nominal fire resistance for floors protected by a 15mm minimum thickness lath and plaster ceiling and min. 37mm joists at 450mm max. centres of modified 30 minutes (i.e. 15 mins integrity) - but this only generally applies to domestic dwellings and there's nothing for walls.
There's loads of free info. on the internet from companies who'll flog you stuff to upgrade this type of construction (e.g. British Gypsum; Envirograf etc etc) and there's also quite a few info. sheets info from local authority building control if you do the right searches.
