For the past ~10 years I've cycled between 5,000 and 8,500 miles per calendar year. 5,000 seems like the bare minimum in order for me to maintain my sanity; any less than that and I would go stir crazy. 8,500 required a conscious effort to "get in a few extra miles" every week, which started to feel like a chore some days.

At no point did my fitness seem to suffer nor improve noticeably regardless of how many miles I rode...because regardless of whether I'm riding 100 or 200 miles per week, I'm always taking rest/recovery days when appropriate* and I'm also not specifically cycling in order to promote fitness, that's just a fortunate by-product of an activity I do for fun.

*(Which is why I'll probably never ride close to 10,000 miles/year until I'm retired.)