How often to change oil when I have an engine Oil Life system for 2007 Chevy

Silverado ? I have a 2007 Chevy Silverado with the engine oil life system. The system shows you what percentage of oil life you have left. When this gets down low I thought a light would come on and tell you to change the oil which would give you about 600 miles to get your oil change. I have never taken my truck to that level. The dealership says I should change every 3 months but than isn't this contradictory of the engine oil life system. Can someone explain this oil life system to me without trying to sell me an oil change. I'm confused, thanks in advance