Def: keep the length
In other words, this handy little expression is what you tell the hairdresser when you only want a trim. Garder la longeur—oui, ca marche. Non?!
So why didn’t he garder la longeur?? I (regrettably) tried yet another new salon and now I’m missing six inches from my hair. I’ve learned my lesson. I will only go back to a Parisian salon with a bilingual friend.