basically they just run them until they're happy that they are stable enough to be released to the public
The 1164 and 1165 that have just been released were only started in beta about a week ago but as most had no problems at all then they went through very quickly. A lot of the large Double Gromacs are still getting EUEs all over the place so they are taking a lot longer.
Basically all you have to do when testing is to keep a close eye on your machines, check back through the logs for any error messages, and report with enough detail and information to enable someone to work out what's going wrong.
In the case of EUEs you report the Project: run/clone/gen numbers along with a section of the log and brief details of your machine, one of the mods will then look up the specific WU and tell you if it's already had several EUEs from other people or if it's unique to you (and thus a problem/bug with your rig/client)