We hate ending buys early so this should be seen as atypical of group buys. I think most of our seasoned community members can attest to that.
Our suppliers generally tell us "sell as much as you can". Obviously, there's some type of upper limit to that number but we typically aren't told what it is. I can confidently say that this sold well beyond expectations, partially due to hoarding but mainly because the online Magic community appears to mobilize itself very well.
It never hurts to join a buy earlier rather than later, especially since we don't actually charge you until the end of the drop date (in the RtR situation, Monday Jan 20th), and we charge everyone the same (final massdrop price) regardless of when they joined the buy.