I'm also under the impression that they never completely finish. There's always more to be found, there's just diminishing returns. And the worse your xenology team is, the faster the returns diminish. Imagine it like this:
A bad xeno team is like a bunch of careless archaeologists. Yeah, they manage to dig up some stuff, but they also break a lot of stuff, and make it harder to find more. A good xeno team would be more careful, break fewer artifacts, and generally not ruin the site.
At least that's my understanding.