How long is one year?

As stated in the release notes for version 19, and assuming it has not been changed since:

... one year sentence = 120 game hours (5 days)

In real time that is 2 hours because 1 game hour = 60 real seconds, but only for a small map.

If you look in the save game file for medium and large maps a TimeWarpFactor is added, with a value of 0.75 and 0.5 respectively, which changes how fast time passes.

Therefore:

            Game hour Real minute    Game year Real time
            --------- -----------    --------- ---------
Small map:      1    =    1m             1    =  2h 
Medium map:     1    =  1m:20s           1    =  2h 40m
Large map:      1    =    2m             1    =  4h

So when played at normal speed one game year can range from 2 to 4 hours depending on your starting map size.

As to the increased speeds on a small map, the >> speed is about 30s to one game hour (2x) and the >>> speed is about 12s to one game hour (5x). So you can divide down the times given above as appropriate.