[@Savage] Right, let me just steal some information from the Deathwatch rulebook...

[quote][b]Nutrient Recycling:[/b] Filters in the armour capture and purify body waste, re-constituting it into a nutrient solution that is intravenously returned to the body. This can sustain a Battle-Brother for long periods of time, but not indefinitely.[/quote]

Looking into the Rites of Battle sourcebook, the nutrient recycling is wholly functional for one week whilst wearing Mk. II and Mk. III suits, and for two weeks whilst in Mk. IV and above, becoming progressively less effective over intervals of roughly the same time as its initial state of full sustenance until the Marine eats some food. In DW rule terms, that basically means "take Toughness tests every 1/2 weeks until you eat some stuff, take levels of fatigue if you fail, and the test gets harder every time it's taken until stuff is eaten", though for the sake of this RP, it shouldn't actually matter, especially since you're mostly controlling your own Primarchs.