I don't think there is anything wrong with longer mash times, as long as you can maintain the temps for starch conversion. In any event, you should get most of the conversion in a couple of hours before the enzymes deactivate or the temp lowers...I would be confident that the temp will not drop a lot.
Make it so!
Secondary: Doppelbock, DIPA
Barrel Aging: Flanders Red
Bottled: Ginger Mead, Scottish 80-/
Kegged: Belgian Golden Strong