Many stay constantly in the bog. However, I will let some of my Pereskiopsis sit outside of the bog as I don't have enough temperature controlled bog area at the moment. So, some will indeed be rotated in and out of the bog as it were. I do try not to let them dry out completely however. I had one scare when I first started this bog project of mine and that scare occurred when my Pereskiopsis started to appear shriveled up just a bit after several weeks in the bog. I immediately took them out of the bog and then replaced them back into the bog after that initial scare. I haven't had any problems with it since then.
I also let the water level fluctuate a bit, but I try not to submerge the leaves and if they are submerged I try not to submerge the leaves for more than a few hours as that can indeed spell trouble.
Here are some more bog pictures as they appear today.
Please note, while I have been experimenting with Trichocereus in the bog on their own roots, I rarely leave them submersed past the soil line for more than 12 hours and most of the time they simply sit in water 1-2 inches deep at most like you see in this last picture.