App developers with apps in the Google Play Store can now give users in-app purchases or a paid app for free with a promo code.
Google on Thursday enabled support for promo codes in the Play Store. There are limits, however. Developers can only create up to 500 codes per quarter. Both in-app promo codes and paid-app promo codes count toward that limit. Additionally, unused codes don’t carry from quarter to quarter; if there are unused codes at the end of a quarter, they’re lost.
Promotions can last up to a year, according to Google.
Source: Google Developer Console Help