Here's an interesting perspective...

USGS NEIC database

1-1-73 to 3-9-11 = 13946 days

  8+ 29 480.9 days
  7+ 523 26.7
6.5+ 1654 8.4
  6+ 5147 2.7
5.5+ 17183 0.8 19.5 hrs
  5+ 61217 0.23 5.5

The list shows the number of quakes greater than a given magnitude occurring globally since 1973, and how often on average they occur.

If you want to predict a quake somewhere on the planet (no location given) then you must have a time frame MORE PRECISE than the average rate, and done repeatedly. For example, a 6+ quake would need to be predicted with an accuracy of at least 2.7 days.

Of course, location specific predictions would decrease the temporal specificity required to beat the average.

Brian