How long are Steam Keys normally valid for?

I was thinking on buying Sono Hanabira on Mangagamer (Potential NSFW Warning for adds) to support English localization of Shoujo Ai/Yuri titles, but much to my disappointment it's digital only so i would only be buying a Steam Key.

Since i do not have Steam installed but i do have an account (uninstalled Steam because it was more or less useless to me) i was wondering how long a Steam Key is valid for, if it ever expires.

Steam Keys bought from Steam never expire - you can wait as long as you want; even if the game is removed from the Steam store the key is still valid. However, there is no guarantee that Steam keys bought from third-party sources will remain valid for eternity. It would be better to use the key sooner rather than later if you do decide to buy it.