They appear to be experimenting (or at least they were until very recently), but yes, on Friday 7/7 the first set was 8:10pm. I rarely saw them step out for a break, strangely - they appeared to stay in the land pretty continuously, with an "entrance statement" (i.e. standing on the boxes + pyro shots) every 30-40 minutes or so. The pyro near the train only seems to go off once per "set." If you want to see it, listen for the train whistle, but the crowd control TMs trying to corral guests for the Dark Arts show can make free movement challenging.
All in all, the Death Eaters add a lot of life to the land. Kids and adults alike were delighted. These guys should really be out all the time during the busier seasons. Couldn't hurt to actually advertise them (though if they did, I think they'd need to double the headcount).