While it is far too early to discern a reasonable pattern in consistory timing - it would be down right boring not to guess anyway. So here goes.
First, some background: there have only been 2 under Pope Benedict XVI so far. One in 2006 and one in 2007.
On 24 March 2006, 12 new cardinal electors were created along with 3 non-voting cardinals. (Summary of the consistory is here.) This brought the total number of cardinal electors to 120.
On 24 November 2007, 18 new cardinal electors were created along with 5 non-voting cardinals. (Summary of the consistory is here.) In addition, Pope Benedict XVI indicating that he had intended to name a 6th non-voting cardinal, but that he had died shortly before the announcement.
It should also be noted that Pope Benedict XVI indicated his intention to abrogate the 120 limit by one at the announcement of the last consistory. But the death of Cardinal Hamao occurred between the announcement and the consistory itself. Thus with the new electors, the total stands at 120 electors.
The 2 consistories were 20 months apart. If he holds to that timing, the next one would be roughly summer of 2009.
But if Pope Benedict XVI continues to try to hold to roughly 120 cardinal electors, there simply won't be many slots available at that time. That is why I don't think there will be another consistory (assuming B16 remains in good health) until 2010.
Assuming no one dies, next year (2008) 3 cardinal elector spots open up. In 2009, an additional 4 slots open up. But, if there was a consistory in late summer 2009, there would only be 6 slots available.
That changes in 2010, with 11 additional slots opening. To break it down a bit more, by the end of March, 2010 - there would be a total of 11 slots open. But by the end of 2010, that reaches 18 total slots available.
So, that's why I'm predicting the next consistory will be held in 2010, sometime after the end of March. A mid-summer or fall consistory seems most likely. And if I had to be more specific, the last week of November seems like a good time.