I win more often against opponents when I play with a 4-minute timer, and, conversely, I lose more often when I agree to a 6-minute timer.
I wonder why that is.
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What could this be for? :)
Lately, I've played a few games with unlimited turns, and honestly, I enjoyed it. It's not too long, and it's not stressful. I think that among players who know and respect each other, and each other's time, this is perfectly normal and acceptable. IMHO.
Therefore, I vote for the minimum possible turn time. In this case - 4 minutes.