Ah, canoeing the Ardeche, this should be my specialist subject. I spent a summer there guiding (adult) PGL groups over a three day descent of the river.
In my opinion, both sets of answers are right. If you’re fit and have canoed a bit, you can run the river in a day fairly easily. Your upper body will be sore afterwards and mostly you will have paddled to make the distance, especially if the wind is upstream over the last stretch. Having worked for PGL while I was at uni, I went back and ran the river in a day with my wife some years later.
But a three day descent is a different beast. It is more of a holiday on the river. You have time to play a bit by the rapids, do the various jumps off rocks along the way (including one from inside the Arc), and chill over long lunches and late afternoons on the beaches. If you’re the type of person who likes ‘go, go go’ it will drive you mad. But if you prefer being with a guide and a group I’d suggest it will be great fun.
And one other advantage of a three day descent is escaping the crowds. By far the majority of people run the river in a day, so they need to set off in the morning and keep going to make it in one day. They end up in fairly big groups on similar sections of river. Three day people can leave later on the first day, they then set off on the second and third days ahead of the one day crowds. For perhaps 50% of your day on a three day trip, you only have other three day people around you.