Once an hour sounds about right. Sometimes it's once every 2-3 hours, but I've had it open more than once in an hour as well. To me it mainly depends on the guard animations. If they both duck, you stay at the MF for a good second or two and then go behind the wall without being caught, there's a pretty good chance it will open. I've had it open a few times on SA as well, but none of those runs were WR pace anyway.
Facility Agent 0:45 (bT fail, bT complete, 2 in a row)
The first one should have still been a 0:45 if I left 0.1s later since the music sounded pretty good I think. Had quite a few 0:44 pace runs, but all had bTs. Might try to dupe it later because it's a fun level and having a 0:44 in colour would be nice.