This question comes up a lot.
My first answer would be, long enough to say what needs to be said.
Long or short, doesn’t matter.
If it delivers value to your audience, they will read it word for word.
Focus more on clarity of the message, so that everything is easy to understand.
My second answer, if I’d have to give you some exact numbers, would be to keep it usually around 300-500 words.
But remember, rules are made to be broken.
If only a badass image with a headline and CTA will do, do it.
If you have to write a bit more to really sell your offer, do eet!
As long as it’s interesting, clear and delivers value, it will get read.