What is the duration of free version of Mailgun

What is the duration of free version of Mailgun???
Because I logged in to the Mailgun yesterday I am now executing the code and it is showing
Invalid api key. But I checked the api key and it is correctly entered. Please help!!!

@dheerajswaroop15

Mailgun allows you to send up to 10,000 emails/month for free in their free plan. If you exceed the limit of the free plan in a month, then every 1000 emails cost 0.50$. The limit of 10,000 emails is more than you might need if you are a small business or an individual.

Hope this help !

You get 5,000 free messages a month for 3 months. If you send over this amount during the first 3 months, additional messages are charged at a rate of $0.80/1,000 messages