Notice after you press Y, the planet information screen on the right will tell you if the scan of planet reveals if it is "Rich", "Moderate", or "Poor". Rich planets usually give about 10+ good scan for ores. Ore yields vary from 1,500 to 3,000. Moderate will give about 8+ good scans, but around 35% to 55% yield compared to rich planets. Poor will give 5+ scans with yields 5% to 25% compared to rich planets. Note: If there is an anomaly on the planet (usually a rich planet) and you land before you scan, usually the Star Map on the Normandy will put you in a strange location away from the anomaly planet. Go back and rescan to get your ores.
For slightly faster planet scanning, put the scanner all the way to the right or left and rotate the planet the same way. The planet will spin quite a bit faster that way; simply push both sticks in the same direction. Start at the top of the planet and "peel" it like an electric apple peeler.