How long should my sales copy be to convince people to buy?

This is a very common question, and there's a very easy answer...

You copy should be as long as it needs to be, and no longer.

That's all there is to it.

Now, most of the time, longer copy gives you more space to make a solid case for your product. That can be particularly important for a product with a high price point (you have to overcome more potential price resistance).

But don't assume that longer copy will sell better. Sometimes shorter copy does a superior job, because you're not making people read a lot before they decide.

This is a place where testing is a must. Write copy that seems long enough to make your case, then test it to see if making it longer or shorter sells better.

That's the only way to know for sure.