I'd say the cap for the "average" person is around 4-5, not to say they can't go higher. The whole thing about "realistic" is that if you just do things in your languages each day maintaining really isn't hard at all once you're already at a high level.

For the "average" person the cap won't be whether they have enough time or mental capacity but whether they want to dedicate enough time to it and whether they can really step out of their comfort zone and find things they enjoy. At the end of the day getting and staying fluent is more a measure of whether you're being a bottom feeder for your entertainment or whether you constantly try to find new entertainment and study material. For me I'll always have youtube videos to watch, books to read, articles to read.

Of course, this all depends. The "average" person is a complete retard so we have to assume we're talking about average language autist, otherwise the answer is 0. I know a few people that learned a hard language like chinese to a c1 and forgot it because work didn't require it anymore.