The key you purchase is not time limited and can be used in the future - so long as the publisher doesn't remove the game from Steam.
It is important to note thought that once you purchase your keys then they are your responsibility and should be kept secret.
If another person obtains your key and adds it to their Steam account then the key can not be used again.
Therefore we recommend that you do not share your key and apply it to your Steam account as soon as possible to ensure that it is locked to you. Always ensure that your fanatical.com account password is secure - it should be unique to fanatical.com, so that leaks from other sites don't compromise your fanatical.com account) and sufficiently complex to not be easily guessed.