At least once a week. I commute 120 miles round trip one day a week, 180 miles 1-2 times a month, and 300+ miles 6-10 times a year.
I also have 3 kids, a dog, and don't live near a city. If I lived in a city dogless and childless something like Zipcar would make much more sense.
BTW: The "transaction cost" (time) of frequent car rentals is a non-starter. One of the things I hate about renting a car is that every time it's a different car, some I like, some I don't. I just want to pick out a car I like, have it ready when I need it, have it set up the way I like it, and have my "box of crap" in the trunk where I know where it is.
I don't want to navigate through a young ambitious salesperson trying to sell insurance, or the AI car scanner charging me hallucinated wear and tear.
How many of these long trips are you making? Depending on number might be more economical just to rent for that.