Collected Thoughts

The thing on the culling seems to be that the Space Vampires (or, um, Wraiths, as they prefer to be called) can go a very long time between feedings, basically sleeping until they get hungry, at which point they descend on a given planet and gorge themselves silly. This happens less often than once per (human) generation, and IIRC has meant up to half the population being culled in a few hours.

The SG:A team has interrupted this slow cycle, and pissed them off in the process (hey, everyone's grumpy when they're rudely awakened, right?), and it seems they're gradually rousing themselves. The Wraith are, though, very patient by human terms, and seem to be taking their time in massing a large force.