I did some quick rough math, and came up with the following. For a bitaxe to find a block once in each interval, you'd need somewhere around the listed number of bitaxes on the network with the current hashrate.

1 block per day - 5 million Bitaxes

7 days - 700 thousand

30 days - 160 thousand

1 year - 15 thousand

How many bitaxes are out there now? Maybe a few hundred to a thousand or so? And we're only adding more.