The above picture emerged over at Symbian-Developers.net that shows something that looks like an internal memo from Nokia regarding software updates.
This memo is kind of old, so the info on it may not be as accurate as it can be, but nevertheless we have an estimate for Belle FP2, or Symbia Donna if you will, and that is week 36 of 2012.
Now don't get overexcited because that is the estimate when Donna will reach software approval, which could mean a few weeks to a couple of months before it hits the market. Remember that Belle has a October 2011 timestamp when it hit the market in February.
In my opinion, this update will only reach last generation Symbian devices such as 603, 700, 701 and 808, with the first generation phones such as N8, C7, X7, E7, E6, C6-01 and 500 reaching the end of the line with Belle Refresh.