How long is too long to spend a vacation at a chalet with your in-laws?

My sister, who is 55 and lives in Europe, has a new boyfriend and is coming to Canada with him this summer to introduce him to the family. She wants to rent a chalet for three weeks and would like her siblings and parents to stay there full-time for the entire visit.

I personally find the stay too long. For me, 3 to 5 days is enough. After that, I start to get bored, and I also believe it might become uncomfortable for her boyfriend.

What would be your ideal length of stay with family at a chalet?