I got mine at 2:56 am. This was a few months back, and I hadn't been leveling my pets very dutifully. I heard there was going to be a server restart the next morning, so I went to park myself in Felwood, but then got worried my pets were too low level. I spent some time battling the tainted rats, cockroaches and moths in the zone to get them high enough, then flew back to the Minfernal spot to be in position for the restart. To my surprise, there was a Minfernal waiting for me. Common quality, but he's since been stoned to rare. So I didn't have to wait for a restart, and I got to sleep in the next morning.
Aside from the pure dumb luck method (which I admit my capture was), there's a comment on the Minfernal page on Wowhead that details how you can actually battle the ones that show up while you're on a flight path, before you land and the CRZ kicks in.