DS is just 3 and almost 40lbs. He only wears dipes for sleeping (at which time he has many layers stuffed in), and we just stopped using the mediums we had mixed in with our larges. We could use one in a pinch, especially with only one prefold stuffed inside, but the tabs were stretching too much (the outer fabric). So, I think they have a super long life in mediums....we'll definitely get a bunch more when #2 comes along.
My DS wore the mediums from 6 weeks old until about 21 months. At 21 months I switched to using petite toddlers most of the time because the rise got too short, but he still wears them meds occaisionally at 23 months and 28 pounds. He was 10lbs 8oz at birth and probably about 12 or 13 when I started using them at 6 weeks. These things fit for-ev-er for us.
My 7 month old DS is in the mediums on the smallest snaps. They are still a little big on him, but have not leaked. He is 17 lbs. and pretty long and lean. I expect to get many more months (maybe years) out of them.
Around 16 months my DS started to outgrow the rise on the mediums, though he still had a few more snap settings left. We went into the large and they are barely tight enough on the tightest snap setting.
I recently switched to cloth with my 13 month old. I bought FB's large, but he could have worn a medium. He is 22 lbs, but his thigh measurement is 10", which is too small for a large. But the diapers are so good that we haven't had any problems with leaking!