@General T. Winters (3 September, 2016, 2:01 PM UTC):
If you mean the one in the photo, then a) A couple of days ago.

The eggs haven't hatched yet, and they don't appear there instantly!

Twas answer ‘A. A couple days ago’

Why?

You may notice that there is a very large and intricate web crafted between the rock and the tent.

That tent was set up when camp was set up. By live, humans.

Mutants came and attacked.

Flys and other tiny scavenger life where likely attracted to the faint traces of flesh, which in turn attracted the spider, who preys on said small life.

Of course the spider cannot spin the web instantly, so it may have taken a little while as well.

As in, perhaps two days, but likely not in one

That means that the camp was likely set up a couple of days ago~