I wear my maxis just touching the tops of my feet. That means that like pants, you need to think about what shoes you want to wear with them. I like them just touching the tops of my feet because you get the unbroken vertical line for height (I'm short), but the skirt is not so long that you trip over it.
I think that if you are taller (I am only 5'2"), you may be able to get away with a shorter length, like to the top of the ankle, and then you can showcase some shoes. But for shorter gals, I think that the longer length looks better. I have a wool jumper (American usage of "jumper" here) that comes to the top of my ankle, and it looks kind of dumpy due to the length. I have to wear it with black heeled boots to make it look good.
I had to have some of my maxis hemmed by my tailor, and he said that he thinks just touching the top of the foot is a good length. FWIW, he told me he used to do alts for Nordstrom for 11 years before opening his own shop. So he seems to know his stuff.