It was both 12 and 24 at different times. 24 worked better because of time zones.

I don't think 7 days has worked out in a useful way (the later portion of the 7 days has mostly just become a way to hide milking rewards, as was the 30 days when that existed) and would be supportive of going back to 24 hours. However, the vote recharge and mana stuff also needs some lockup period to avoid similar issues of vote then move stake and vote again.