Given that lower-end Macbook pros are still dual-core, and that the new Macbook refresh (if it happens at all, not everybody is positive that it won't just be killed) that would be about to happen won't out-spec a mid-cycle Macbook pro, you're looking at a timeframe of at least 13 months, and probably something between 2 years and never.
2 years because that's probably how long it will be before quad-core is the smallest Intel they'll put in a machine. Never because by then there's a decent chance Apple will either: 1. Kill the Macbook line in favor of the Airs and low-end Pros, or 2. Change processor philosophies.