Depends what you mean by long and what benefit you are looking for. I’ve never seen audax length rides as improving fitness, more a test of yourself, your bike setup, and your equipment / clothing and food choices.
I’ve got an easy four hour ride planned for Saturday. I have no illusions that it’ll make a difference to my fitness for LEL. It’s more just going out for a nice ride, keeping the legs turning over, plus a final check of the fettles I did on Sunday. I then have a bit of intensity and an easy two hour ride next week for more legs turning over. But nothing more, nothing that’ll generate significant fatigue.