Almost all the keys you purchase from fanatical.com are not time limited and can be activated in the future - so long as the publisher doesn't remove the game from Steam.
On the rare occasions when a key does need to be activated within a specific period of time or by a certain date we will clearly indicate that within the game's description on our store page.
It is important to note 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 cannot be used again.
Please note though that the longer you leave a key without activating it the less likely it will be that a replacement is possible should the key be defective - replacements may not be available for keys delivered more than 60 days ago.
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.