I'm a physical therapist and tore, not sprained, my mcl about two years. the mcl repairs itself and rarely requires surgery. Technically you can snowboard without an mcl and you really won't notice a difference, but it will hinder you in other aspects of your life especially as time goes on. I took a solid 3 months off of anything that would hinder the mcl regenerating. If i was to continue snowboarding it wouldn't heal properly and GREATLY increase the chance of it happening again. if it remains torn, the other ligaments of the knee are stressed more increasing the chance of injury.
if you're experiencing pain its more than likely a sprain, there is very little pain associated with a tear....oddly enough. ice and alieve for the pain as long as you need. then i would strongly recommend staying off it for at least 2 weeks.