How far in advance to make reservations for caves in the Dordogne

This might help: https://tickets.monuments-nationaux.fr/fr-FR/produits-seances If it gets wonky like it did for me, try selecting Font-de-Gaume on this page: https://tickets.monuments-nationaux.fr/fr-FR/familles?site=2117969240920404140

Be sure to pay attention to the details on the left side of the page. When we visited there was one woman who absolutely did not get it that she could not take her purse into the cave. There was an alcove at the entrance with cubbies for possessions. It was gated and locked while we were inside.

BTW, we also enjoyed the Lascaux II replica we saw. There are more options now than then. I'm sure they are equally if not more amazing: https://archeologie.culture.gouv.fr/lascaux/en/practical-information

For us another interesting place to see in the area is a fortress castle built into a cave. It's La Maison Forte de Reignac: https://www.maison-forte-reignac.com/

As you drive along that valley you can also see cliff dwellings like La-Roque-Saint-Christophe: https://www.francethisway.com/places/roquestchristophe.php

We also stayed in Sarlat. That was way back in 2009 (yikes!), but we found the town lovely in the evening with its gas lighting. And there were good places to eat very near Les Cordeliers, our B&B on the town square where the market was held. Our room was on the top floor with a view of that square: https://lescordeliers-sarlat.com/en/accueil-en/

Elizabeth,
We spent 10 days in the Dordogne last June (first half). Font de Gaume was important to us to visit, so I monitored the website that Lo referenced daily from about two months out. The tickets we wanted for the tour in English came up May 4 for June dates. I suggest you begin checking at the beginning of March and snag them as soon as you see the date open, as there is usually only one tour in English per day. It was my 9 year old granddaughter's favorite, of all the things we saw, because it was the true and real drawings and cave. I have a trip report posted, dated June 27 titled "Just Returned From Four Weeks in France...Continued" if you want to read more about our trip. You will love the Dordogne region. I have been to Lascaux II (in 2001) and Lascaux IV. Both reproductions are well done, but IV has more to offer. I reserved our visit and tour there as well, because I wanted to be sure to get one in English. (And there were seven of us.) There are more offerings per day there, so you can probably reserve a little closer to your stay. Walk-ups seemed available when we were there, but not necessarily in English.
I recommend seeing them on separate days to avoid information and awe overload. If we had had more time I would have visited one or two more, but there is so much to see and do in the area that we had to pick and choose. Don't miss Castelnaud la Chapelle and Beynac Castle. We went kayaking on the river.
April may be too early for kayaking, but the river and valley are beautiful. I advise seeing the castles on separate days to avoid the dreaded overload. You can easily plan on two things in one day, but varying them is, IMHO, more satisfying.

FYI, the prehistory museum in Les Eyzies was closed on the Wed.that we went to Font-de-Gaume, and we really wanted to go there after the cave. Try and visit there if you can. I would check their schedule.
Have a wonderful trip!

Ticket sales happen anywhere between about a week and 3 months before the date of visit. They also tend to open on different dates for the different caves. In 2022 we had our Font-de-Gaume tickets 8 weeks before our visit, but the tickets for Abri Cap Blanc were only available about 2 weeks before.

I would keep checking every couple of days, but if you start getting nervous about being able to book send an email to the tourist office at Les Eyzies. They can't sell the tickets to you, but they may be able to prompt National Monuments to open booking. Emailing direct doesn't seem to have any effect, they're a large organisation.