I have tried to measure the change in Elo for Stockfish, using Stockfish 2.3.1 (which I believe was the original "master" or reference version) as the reference. For my measurement, I have used the generic Linux 64bit compiles available at http://abrok.eu/stockfish/ . For each month, I downloaded the last development version available on the 22nd day (the chosen day due to SF 2.3.1 being dated 2012/09/22). Each version played each other 100 times (50 openings/reversed colors). Also included are the official releases.
CPUs: Xeon L5420
Time Control: 60" + 0.1"
Hash: 128 MB
Openings: 50 positions 2 moves deep (a subset of the openings used in round 2 of TCEC)
Adjudication: resign @ 450 cp for 4 moves; draw after 120 moves if score < 50 cp for 4 moves
Total number of games: 159600 (games available upon request)
Note: There is a distortion in the Elo ratings for Stockfish 3 and Stockfish 4 due to 7 time forfeits (all by Stockfish 3 when playing Stockfish 4) that have not been replayed yet.