I've once tried a campaign as Spain with 6 months/turn.
In the beginning, it seemed to be ok, but the longer I played the more it got boring.

I like to have some time to built up and turtle a bit, but I'd never reached the later ages and seen the Timurids or the new world. You need some development to spice things up a bit.

For me, I found 1 year/turn to be ideal. Still, it's odd, but I don't bother as much anymore as I did at first.