For the general question, I don't think there's a set "too long". It depends on the reason for the trip, frequency of visit, reciprocity, personalities, living situations, and more. I once stayed with my parents for about 6 weeks when I was evacuated from the country where I was living. They were thrilled to have me home for a while, and to provide shelter during a terrible time. I'm planning a visit this summer and since it is a long, expensive international flight, they want me to stay a while. I threw at the 10 days figure and my mom seemed disappointed and managed to sneak the phrase "2 weeks" into the conversation. So clearly, long visits are fine. But they have tons of space, I'd like to think I'm generally a low maintenance guest, I do this only every 2 years or so, and their lives continue while I'm there though an occasional tennis game might be skipped or bridge might be moved to someone else's house, but that's entirely their choice and mostly they keep their commitments).
In this particular case, I'd likely decline entirely. I feel like it's easier to just say that it won't work than to say "well, you can come, but only for 5 days". I'd consult my husband since it's about his family, but that would be my initial inclination. What's the reason for the trip? Are they coming to see you specifically, or are they using you as a hotel while they visit your area or just get away from home for a bit? Regardless, you could include in your email or text some recommended hotels or vacation rentals. "Sorry, Jane, but a stay at our house this summer isn't going to work. Bob's Motel-o-Rama is just down the street and I've heard it's nice and affordable, or here's a link to 3 nearby vacation rentals. Hopefully one of those will work for you! And let us know when you'll be in town so we can schedule a few nights to get together for dinner or something!"