Here's an informative post I remember reading.
"By and large, those tests are s raving a dual purpose: find out creative problems and attempt to resolve them prior to release, and discover marketing options and demographics. The production team is actively seeking information that may help them market the film on release.
Additionally, there are endless test screenings before that test screening. These are typically more ad-hoc and informal- screenings with advisors, mentors, friends, and other parties who might have valuable feedback during the editing process. These are the screenings that produce the biggest changes, as the edit itself is in process and can be greatly influenced.
It all depends on the production, the studio or production house, and what their marketing schedule is.
For marketing test screenings, you may have them 1-4 weeks ahead of release. Pretty quick turn around.
If you're looking at a fine cut and giving feedback, as this post suggests, that can be anywhere from 2-6 months ahead of intended release for typical films. Sometimes up to a year. A major film with a huge workload of SFX will go in to test screenings even before the wrap of production, such as happened with World War Z; the goal being to ensure that they aren't going to waste huge amounts of money on SFX that are in scenes they'll want to cut. World War Z, however, had another problem: they learned that audiences hated the entire last third of the film, so they decided to reshoot it entirely."
I remember reading about a WW test screening that had graphic models in the fight scenes, so here you go
