From the perspective of playing to win and honing one's skills, a 6-minute limit is quite reasonable. I personally play with a 4-minute limit, but I usually agree to 6 minutes if my opponent suggests it. The maximum 6 minutes are not required every turn, but only for some of the most important days – transferring an army from one end of the map to the other. This is a safeguard for those who haven't mastered the art of rapid clicking.
However, we shouldn't forget that there is real time, real-life matters, and no virtual goal can compete with real-life goals and activities. And 6-minute games are still longer than 4-minute games. Plus, there are those dreaded restarts. Therefore, I am strongly against formalizing a 6-minute limit. Such a limit should only be used by mutual agreement.