Hi Tom and welcome to WAB.
No problem being in France, someone has to live there!
Nyger is supposed to be the seed of choice for Goldfinches, if your getting them visiting your Garden your at least half way there. I'd just persevere. Keep the feeders going and in time they will no doubt start to feed.
As for when to stop, the general advice is don't!
I keep mine going all year round. Even when the conditions seem ideal for the birds to feed from natural sources, they still come to the feeders especially if there's young birds to be brought up. The birds aren't daft, they feed themselves on my food and the young on natural food.
I can't answer your second question, I hope someone else will.