Having been "camping" this thing in gusgen mines today, while doing the dishes/housework, checking back every rough half an hour, I'm pretty sure the time is variable.
Occasionally, I'd check back in 25 minutes, and the ??? will have repopped in front of my nose, and sometimes it wasn't there yet in 30, but was there when I sat down to go check spawns 5 minutes later.
This was using the ingame clock, so pretty accurate.
However, I timed afetr NM death, not after touching ???. Maybe that matters, I don't know personally.
That said, I'd put time anywere from 25-35 minutes.
Nitpicky, 30 min's is probably accurate enough, but it's not exactly that, indeed.