Timed-entry reservations in France

I think it depends on the venue.

I’m most familiar with timed entries at Paris museums. For the Louvre go ahead and look at the calendar on the ticket booking page and see tickets look like for the dates you have in mind. If you are considering the museum pass look at those days vs just buying outright as the ticket numbers are different.

https://www.ticketlouvre.fr/louvre/b2c/index.cfm

For each venue you plan to visit I’d go in to their ticketing website and see what the calendar looks like.

Go to each website and start the reservation process, picking a date not far from now. That will tell you what availability looks like. Check weekday vs. weekend, or holiday period, if that's relevant to your timetable. Don't go beyond that with the reservation, just back out. Then check again periodically. You'll figure out when time slots start to fill for each place, and when you'll need to commit to a particulalr day and time. It will vary between sights of course.

Thanks, Pam and Dick. I have been doing what Dick recommends for the last week or so, and it does seem to provide good insight into how mad a rush I need to be in.

Nail down the Louvre when you know your dates; it is one that can book up several days out. In our not recent experience the chateaux of the Loire are not crowded and you don't need to reserve -- but it has been awhile.