Your driving rule

There is long in terms of distance, and there is long in terms of time. In Europe windy, hilly and narrow roads can make a relatively short distance into a long ride.

Two hours.

About 1 Hour is long enuf for a turnarounder otherwise I plan on staying the night there.
And another gd ques. max travel time to destination by car no more than 3 hrs. At 4 hrs I am wiped out/fatigued and feel like I have not had a vacation day. Pacing oneself is very imp.

I'll consider two hours ,and I would probably go for one and a half hours. But one hour or less is ideal.

2 hours max to one particular site, but we may spend the entire day driving to multiple sites before heading back to our base. 6 hours (driving time) for our longest drive to our next location - assuming that we will actually take most of the day when you include lunch, short sightseeing break, etc. And for both of the above we don't do 2 long driving days in a row. So the day we drove to Assisi and back was followed by a few days of very close in sites. And our very long drives are followed by 2-3 nights in the one spot.

I'd say two hours each way is the absolute max for me. I think it's better to move than lose more than four hours of your day to driving.

Two hours maximum with as little night driving as possible. Prefer to leave early in the morning rather than returning back late in the evening. Nothing worse, for me, than to get lost and it's dark out. Don't enjoy that after a long day.

If you're on the autostrada 3 hrs is fine.