Are there any known "available for purchase" life spans for expansions for Android Netrunner?
Theoretically?  I don't see them ending production on a specific cycle.  The way Fantasy Flight talks and creates cards, they plan on keeping all of those options available for the future.  The LCG philosophy makes it so that no one person is going to wreck another one because of the super power of money.

Realisticly?  I'm sure FFG will find a time in the future (given that Netrunner continues to run for a long time) where they don't see as many sales of the older cycles.  At that point, I can see the company doing one of two things: they will either roll up an entire cycle and sell it as a bulk set at less of a price (say the entire Spin Cycle as one whole purchase) or they will try and discontinue the set and put it into archive.

All of that being said, my suggestion to you is to NOT be behind more then one cycle of cards.  Right now, if you don't have the whole Genesis Cycle, you need to get it.  Also, if you don't have a few packs of the Spin Cycle, you're a little behind.  And if you don't have Creation and Control, I feel that your deck building options aren't as robust as they could be.  I'm not saying you won't be able to make a deck.  But the newer cycles, C&amp;C and H&amp;P all have some 'golden' cards that are defining the meta of the game right now.  You can still play, but competitive play will be seeing people playing along with the meta.

EDIT:

A few months ago, FFG did mention that they will be cycling out current sets of cards as being tournament viable.  Much the same way that Magic has a Type 1 and Type 2, Netrunner will only have the latest 6 cycles active at any one time.  Meaning that when we get to the 8th cycle, Genesis and Spin cycle will be phased out of tournament play.  This does NOT mean that they'll end production on these cards, but it's a pretty good guess.