The Crater Lake Rim Visitor Center indicates that they open in Late May, and close in Late September. The restaurant in the Lodge opens a bit earlier and closes a bit later than the visitor center, so it may be passable by mid-May next year.
http://www.nps.gov/crla/planyourvisit/hours.htm
That website also has alerts that will let you know the specifics when they've worked it out themselves!
Pro-Tip: Stay at the Lodge if you can - the park doesn't open until 10am(?) but if you're at the lodge you can get the whole place to yourself for several hours before the tourists show up!