My time line:
Newborns- 2 weeks. (8 lbs at birth)
Size 1 - until 2 1/2 mos
Size 1/2 (Huggies) until about 3 mos old.
Size 2 until 4 or 5 mos old. Briefly used swaddlers 2/3. Not very long
Here's the kicker-
DS is 20 lbs, 14 mos old and still fits just fine in size 3.
I stocked up on size 4's. I thought he would need them soon. 8 months ago. The only good thing is that I got Cruisers without drymax. I think the size 4's start at 22 lbs... He has been wearing Huggies overnite size 4s for along time because they are easier to find.