I've play tested for a couple of years but it was not a fulltime job. Every once in a while this company I signed up with would send a mail like "hey would you like to participate in testing Project Y for two hours" and I could sign up.
It typically was online network testing. Connecting to other players, do some co-op, that sort of stuff. Signed a lot of NDA's but got to play several big releases way before they came out which was very cool.