The element won't overheat and it won't be damaged by running for long periods. Also, acquiescing to Mrs B's needs in this way will improve the atmosphere in the passenger cabin
Mine reduce from three bars to two then one on their own, when heated up. So they will not overheat.
I personally just get the seat warm and switch off, my wife would have them on all day.
I personally wasn't worrying about the heater element but the nappa leather. My driver seat is creased a lot more than the passenger's which is as new and after softening the leathet it seems slightly better. I am worried that from that hot going to cold again and again, it could potentialy stress the leather more leading to creases. I could be wrong but don't use mine a lot. Plus Brian WH is correct as the heating goes down gradually to 1 dot.