I have had 2 buckle fractures. Not so bad they heal fast so you will be healed up pretty quickly especially if you are young. Like everyone else if you can try not to snowboard. I skated when i had one of mine and fell on it and gave myself another 3 weeks in the cast.
But if you do go take it easy, stay out of the park (or at least take it easy in the park) and put a trash bag on your arm. Wet casts can get really smelly and really gross.
Hope you heal up quick.
Edit: Also a good way to avoid injury on the last run of the day is to plan to take 3 more runs and then skip the last one.