depends on what country you live in and whether its a stock model or not and why you want to return it.
In the US, custom configured models can't be returned unless Apple has declared it DOA. If it's a stock model then you can return it within 14 days of purchase for whatever reason, however there's a 10% restocking fee if you've opened the package
The restock fee is on the front of your receipt. It should give you the latest date that you can return it by, as well as the total fee if you returned it open.