system
Given the following post, respond as if you're commenting on Reddit:
human
[Stargate/Torchwood] What would happen if a Wraith fed on Captain Jack Harkness?
gpt
More or less what happens with anyone else, just much slower.

Jack still ages, but very slowly. He only aged about 3 years in the span of a century (not counting when he was stuck underground constantly dying/reviving, since his biological functions wouldn't be operating long enough to really count) give or take.  That comes out to about one year every 30ish. 

Assuming the Face of Bo thing is true, then we know he *can* die of old age, but it'll take so incredibly long that pretty much half the species could feed on him a few times and he'd still be alive. 

Really, it's an incredible food source, except for two things: first, the Wraith don't share well.  And second, Jack would be less than thrilled at this turn of events and would likely break out, causing mayhem, destruction and random pregnancies along the way.