My 11-year-old son borrowed a hunk of Buckyballs from one of his friends a couple of months ago (the "hunk" had somewhat over 300 balls). We had them for about three weeks, and could barely bring ourselves to stop playing with them at bedtime, dinner time, homework time, work time, etc.
As soon as we gave them back, we were missing them.
By the way, there were things we wanted to build that would have taken more than the 300+ balls that we had. Two sets (432 balls) would be a nice amount of them.