once or twice a week.
it keeps it even and if you have dirt or sand, it keeps it soft and fluffy

I think that it should be done once a day. You can get contraptions that have water tanks as well, so that when it's evening out your riding surface and keeping your footing soft, it's moistening it as well to minimize dust. A freshly dragged arena is A LOT better to ride in than one that hasn't been done all week in my opinion. Some places even do it more, like the place that I take lessons, I'm pretty sure they do it 2 times a day.

Thanks peeps, where I barn my horse, they hardly ever do that to the arena and while working with my horse she almost had two falls with me. I asked to drag the arena but they said no need too, it's fine. Very questionable why they think that.

I don't have an indoor arena yet. But I wish I did.

It is best to rake it when it gets uneven, and its is easier on some horses legs and hooves.