Varies so much with each baby! My first son was in each size exactly when he should be, and he is still that way. But he has always been pretty much in the 50% for height and weight.
Colton is a much faster grower, though, so we've blown through lots of sizes already. He was in Newborn for about 3 weeks, 0-3 from 4wks-9wks, 3-6 from 9wks-12 wks, 6-9 from 12wks-18wks, 12 months from 18wks - 8 1/2 months, now he is in 18 months clothes at 8 1/2 months. The smaller sizes just went so fast, it was literally only a few weeks in each one! He slowed down a lot though, so at least we aren;t still going through sizes like crazy anymore!