The key you purchase is not time limited and can be used in the future.
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.
For help applying a Steam key to an account view our Step-by-step guide to using your new Steam key.