How does the shelf life of Flixotide Accuhaler containing fluticasone propionate vary between the 50, 100 and 250/500 strengths?

Flixotide Accuhaler containing fluticasone propionate has a shelf life of 18 months for the 50 strength, 24 months for the 100 strength and 36 months for the 250/500 strengths, all when not stored above 30°C.