Let's Encrypt Certificate expiration date

Today I activated plugin Let's Encrypt and I made a test with one of my domain.

The Certificate expiration date now is 05.01.2017

I did not find anywhere if this expiration date can be longer or how to can be longer, or what will be if the expiration date ends.

Can do you give me some info about this?

Thank you very much

Thanks for answer

Thanks Ninos