In general, once a flower stem has doubled in size, it is time to cut it back by one-third. However, if the stem starts to droop or looks damaged in some way, then you should instead cut it all the way off at the base. You can also pinch off wilted flower parts, but ultimately it will be up to you as to when you would like your plants to bloom. There is no need for regular pruning unless the plant appears damaged in some way. The bottom line is that calla lilies are not difficult plants and will reward regular attention with beautiful blooms each and every spring!