The length of a turn, kind of depends on the scope in which you look.
As far as population growth, the average city seems to grow in 5-12 turns. I imagine city growth as a new generation being born, and maturing to working age, thus providing more citizens. So it really has to be about 16 years. So in this scale, I'd say a turn is between 1 and 4 years
Unit training. I'd estimate the scale to be about 1-3 weeks per turn, given that you're manufacturing wapons for the unit, and having them trained at your military academies.
On a tactical scale, battling, moving units and such, a turn is 1-3 days. Leaning towards 1. Generally, battles wouldn't continue on at night, since you can't see the enemy. So I'd estimate each turn of fighting as a single day, after which your troops return and heal.
When it comes to healing, I'd say the scale has to be a bit longer. Generally, I imagine unit healing as a combination of the surviving units bandaging up, and resting while they recover, and reserve units being brought forward to replace those who are dead, or too injured to recover in a reasonable time. I'd estimate a turn as about 4 days then.
On a construction scale, it doesn't really make sense that it takes 40 years to build a granary. So as far as domestic production goes, I'd estimate a turn as anywhere betweeen 2 days, and a week.
When it comes to wonders on the other hand, I'd say a turn has to be at least a couple of months. Didn't the Pyramids of Giza take something like 18 years to build? Envision it on that sort of timescale.
When it comes to research, taking into account that most research is rediscovering old things, I'd estimate a turn as 2-4 weeks.
As a side note, it seems like slower gamespeeds make a lot more sense, since unit speed isn't scaled. The timescale is a lot easier to reconcile on Marathon. But even still, city growth and wonder building really should take a LOT longer.
