I'm not sure about this, because I'm not an eBay seller, but I found this. Perhaps keeping an eye on it would tell you what you're looking to know.
If you want to know how many of your free 50 listings for the month are left, or if eBay is running another free listing promotion, here are the steps to find out:
1) Hover your mouse over My eBay (do no click My eBay).
2) Click on 'Selling' from the menu that drops down while hovering over My eBay.
3) Scroll all the way to the bottom. There should be a section called Listing Offers. Under this section you will sell how many free listing offers you have left.