Most people I've seen say the max is 3 - 3.5 vols of co2. Though I'm pretty sure I've seen Kristen England say in posts on the NB forum that they will hold 4 vols.
I suppose it depends on the quality of the bottles. The last batch I actually bought were definitely thicker and heavier than the previous bottles I'd bought. I thought they felt a bit heavier so I actually weighed them and they were a few ounces heavier.