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EVA suits use an array of consumable resources: oxygen, CO2 filter, nitrogen, electricity, sublimator/coolant water, drink water, maybe something else I missed.
So, if an astronaut on EVA in LEO was unable to return to the craft/station, floating away into space - how would they die? What's the first resource(s) they'd run out of that are necessary to survival?
From what I remember it varies between oxygen and battery. Here's an example from STS-127 where the answer was oxygen.
Both spacesuits are working well today. Mission Control projects the EVA-limiting consumable is oxygen that should last a total duration of seven hours and 30 minutes. The spacewalk just passed the four-and-a-half hour mark. If the crew needs more time to finish the battery task, they could return to the airlock for a quick recharge of the suits' oxygen supply.
ISTR that battery life limited some EVAs, but perhaps battery upgrades have made that obsolete.
For your case of a stranded EVA crewmember, it might depend on what they did. If they just lay quietly, battery would probably be the limit. If they thrashed about in panic, probably O2 or cooling water.
