Hmm... I don't know what the statistics would be, but somehow I have a hunch that I may have spent a larger percentage of my mando-time standing than most— under some fair creature's balcony, of course! (*ekhm, ekhm* I DO mean my wife, daughter, or sister...)
So, when the crossing of legs gets a bit tiring, I revert the mandolin to the position I hold it in while standing (even though, naturally I stay sitting) and simply uncross my legs long enough to loosen up.
Even without those breaks, though, keeping my legs crossed does not bother me; I try not to bear down on them with my upper body weight, which, as Jim writes, would lead to instant gangrene. Hmm... I also seem to cross my legs very loosely, very lightly, as if I could hypothetically stand up in an instant, without any major adjustments in my posture.
I also have a foot-stool from the bygone era I played the guitar. Somehow, however, I don't seem to find adequate reason to use it while playing mandolin; in fact, it feels like the stool would need to be adjusted super-high in order to bring the mandolin up to the right height.
All this, of course, bearing in mind that I play simple repertoire, 99% in the lowest 5 positions, and that I am far from the requirements of "concert" performance. Perhaps its that rough-and-ready lineage of great-uncles playing mandolins on family outings, sitting on any old boulder or tree-stump they could find readily available...
It is not man who lives, but his work. (Ioannis Kapodistrias)