DS1- Birthweight 10lb 8oz, Five times would be 52lbs 8oz. He weaned just before he turned 5 years old. He did weigh around that!
DD-Birthweight 11lb 4oz, Five times would be 56lbs 4oz. She's still nursing but I'm positive she'll wean well before that. She's barely 30lbs at 3 years old so I'd guess that she'll be about 8 or 9 by the time she's 56 lbs!
DS2 was born smaller than DS1 but is gaining at an even faster rate. I have a hunch that he'll wean well after he hits five times his birth weight.