Catching the snitch gets you 150 points, scoring with the quaffle gets you ten points. So you have to score 15 times to match one snitch catch. And the game doesn't end until the snitch is caught.

Fun fact, in one of the pro quidditch matches in the fourth book, the snitch-catching-team actually loses the match. They built their team around their seeker, and basically just banked on him immediately locating and catching the snitch every game, which backfires in the final match of the Quidditch World Cup or whatever, when the opposing team had really good chasers that ran up the score really fast just by scoring with the quaffle.