We've done 2 weeks in France, in a few different places (as in, different holidays - stayed in the same place for 2 weeks each time), which was lovely. Plenty of time for some adventures and outings, but also plenty of time to read, enjoy relaxed cooking, do a lot of swimming (DD did a lot of sailing and I did some on 1 trip which was fun).
We have done 2 weeks in the Netherlands - 1 week at a sailing event and the 2nd at a holiday place in 1 trip. That was also nice but the sailing event was stressful so not particularly a holiday part of the trip.
We are just back from 6 days in Amsterdam (having done a couple of city breaks there before) which was lovely too as it was relaxed and we had time to wander - but I would have liked a few more days to do a couple of days going to other cities by train for the day and a couple of days doing nothing but reading a book.
We've done lots of city breaks (up to 4 nights) across Europe, and I have done a fair amount of travel for work and had a small amount of personal time in some of those (an extra day or a day I needed to travel from 1 place to next but could do that at my leisure and with stops along the way).
We have never gone over in our own car - always rented a car on arrival or been where there are good public transport links. If we had our own car, we would likely do more multi-location trips as we don't mind doing that in the UK and Ireland.
Usually, we are happy to do a city in 3-4 days. Usually, we are happy to spend at least 10 days in a relaxed location where there is decent food (nice local restaurants/takeaways and good shops/some facilities for us to cook ourselves), swimming, walking (and cycling) etc. DD always wants to be doing things (hence lots of sailing - and this year she went to a sports training camp while we had a holiday), and DH can't sit still for days on end but likes to have a mix of relaxation and activity.