As long as you want them!
The longer the content you require, the more time it takes to write. 4,000 words takes roughly 20 business days to turn around.
Do keep in mind if you exceed your monthly word quota you'd be billed overage rates to send through the request and have a writer work on it.
If you're looking for multiple pieces to be completed, our recommendation is to submit it as separate requests so they can be done in parallel by our writers and delivered back to you faster.