Today, the College of Cardinals has 193 members, of which 112 are eligible to vote should a conclave become necessary.
By the end of 2011, that number drops to 110. At the end of 2012, it would be only 97 voting Cardinals.
Pope Benedict XVI has held 3 consistories to create cardinals so far: in March 2006, November 2007, and November 2010.
At the time of each, the number of electors was: 109, 103, and 101, respectively.
This strongly suggests that Pope Benedict's next creation of cardinals will take place next year. So the next question, is more precisely, when?
Of course, that is a decision of the Holy Father himself, but we can consider some of the factors that might influence the decision.
Toward the end of March (as in 2006) might be a good time. There would be 107 electors at that point, providing roughly 13 slots to fill. However, with the early Easter (8 April), this time frame seems unlikely.
But the early Easter does suggest another option: a date around Ascension or Pentecost (mid to late May). That time frame has 105 electors - leaving roughly 15 spots available.
After that are the summer months, but Pope Benedict, like so many Romans, usually escapes the heat and heads to the countryside.
October might be an option, but next year there will be many special events relating to the 50th anniversary of the beginning of the Second Vatican Council.
Another option is the end of November (as in 2007 and 2010), at that point there would be only 98 electors - providing roughly 22 slots.
So with all that laid out, my suggestion of the most likely dates are 1) the weekend of 26/27 May (Pentecost) or 2) the weekend of 24/25 November (Christ the King). Of the two, the former seems the most likely.
The next question I leave to others for now, namely: who will be included?