Here comes the pro answer I have been a kitchen and bath designer for over 20years. The answer is standard 31 new trend past 10 years or so they have been changing over to 36 now you also custom size them to what is comfortable for you. We did ours at 36 for comfort and cost. Hope that helps!
When I did the remodel to the bathroom I set mine at 36" since we are both tall, also raised the kitchen cabinets and sink when I replace them. People were shorter overall when the building standards were made way back when. I was elated when they finally came out with the extend able vacuum wands. Now if they could only make a longer mop handle.