Joe,
Since a lot of the behavior for the little Mercs does not translate up to the larger octos, I don't know that my observation helps but neither Trapper nor my 4 males kept (keep) a den for more than a couple of days. Dens are revisited and trapper seemed to make a regular circuit (counter clockwise) of the tank until she started brooding. Miss Broody, however, chose a den after being released to the tank and never slept anywhere else (until her disappearance 4 weeks ago) even before she mated.
My hummelincki (definitely diurnal - only with us 1 week so far) found a den her first night and returns to it nightly or when she wants me to "go away" difference is she will go in with no one present and not come out until the next day at bed time but the "go away" escape is only temporary and she will come out again when we leave the tank front).