depends on the type. most people have the large mallow with the pink flowers. I cut mine down when it has finished flowering in Autumn. I have never divided it but if I was going to then I am suggesting that Autumn or Spring when you do other perennials. I work on the idea that if it blooms early in the year divide in Autumn, if it blooms late in the year divide in Spring. Generally speaking this has worked for me with no major disasters. There are other shorter and smaller malva, I grew some from seed, but I would treat these in the same way.