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Introducing Steam Families
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It's slightly better than that for the person who leaves. It's a one year period starting the moment they joined the previous one. So if you've been part of a family for 1+ years you can join/create a new one right away.
The slot you occupied however does stay locked for an additional year.
I also have my current setup with found family and as I live close to a country border I cannot switch over properly as I have members on both sides of the border. I understand their intent is "same household", so I do understand why this is the case, still sucks for me though.
After toying around in the beta, this seems to not be an issue anymore as they seem to actually go off accounts now and not hardware anymore. It was quite frustrating in the old system though.