We've been having sex regularly for almost 25 years. For the last 5+ of those we've used the Liberator Throe to protect the sheets - before that I always got the wet-spot! With the Throe for protection - we can change the sheets 1/week.
My partner and i only have sex "regularly" when one of us is visiting the other. We both tend to clean the sheets before the other gets there and then again after they leave. So maybe every couple weeks? We dont make much mess.