I would not recommend this to most people but I go up to 30. The reason I would not recommend this is because most people don’t have the base to support such a long run. My personal take is that, with exceptions of course, you shouldn’t venture beyond 20 until you are comfortable at least with 50-60 mpw. As you work up from there, I would have the long run mileage climb at a slower relative rate than weekly mileage.
My reasoning behind this is that I believe too many people put way too much stock in the long run and ignore other crucial training components. The long run is of course a key component but it isn’t the only thing that matters.