Developers have complained that their updated description will appear a few days before their updated binaries submitted at the same time.
When you update a description, price, or any data like that that will be published in about a day (as soon as the App Store "refreshes"). Even though your updated app appears as a separate listing in iTunesConnect, it shares the same description as your listed app, so there is no way to change the description so it goes live at the same time on the App Store. However, you will receive an email that your update has been approved (usually about a business week) and you can edit the description then so both the update and the description go live at the same time (once again, about a day as the App Store refreshed). Not a perfect system, but definitely manageable.