From the rules: http://www.netflixprize.com/rules

When the RMSE of a submitted prediction set on the quiz subset improves beyond the qualifying RMSE an electronic announcement will inform all registered Participants that they have thirty (30) days to submit additional candidate prediction sets to be considered for judging. At the end of this period, qualifying submissions will be judged (see Judging below) in order of the largest improvement over the qualifying RMSE on the test subset.

Maybe it should, maybe it shouldn't, but Netflix defined it as 30 days from the first submission over a 10% improvement. I'm sure there's some reason why they did it that way, but I don't know what it is.