I would say annually and never over pot your Massie, select a pot which will take the root ball mass without excessive space left around the roots. However you may need on occasions to repot due to growing problems, like your media breaking down, rotting or media crumbling between your fingers or giving off a mushroom smell these signs mean it’s also time to repot. Always remember allow your Masdevallia time to recover after repotting. I usually do mine in early spring allowing it time to recover during the growing season, I try to avoid repotting during the winter months.