The most important thing: don't wait for a promo to use your miles, because point fares stay pretty much stable all year round with Air France. You go on the site, you go to the "Book with Miles" section (in your Flying Blue account), you enter your destination and you see directly how much it costs. It works 24/7, no need to wait for a special date. The classic trap everyone falls into is thinking you need exceptional promos for it to be worth it, so they keep their miles waiting... and yeah, they never end up using them.
Free flights vary depending on distance. A short-haul like Paris-London costs around 25,000 miles in low season, more expensive in peak season. A long-haul is easily double or more. Heads up: there are administrative fees on top (airport taxes, fuel surcharge, it's not huge but it's never zero). You also need to check that your miles aren't expiring - they have an expiry date if you don't use them within 36 months of your last account activity.
Last thing: miles don't always combine with discounts, so a flight that's already cheap in euros won't necessarily be worth it in miles. Play around with the search engine, compare the two pricing options (euros vs miles) on several routes before you decide. And honestly, intercontinental flights in miles - that's where you actually save compared to the regular price.