Hi
What is the average life of each miner (S3 for example)?
That's a very hard question to answer because it depends on so many things. As the first reply mentions, cost of power plays a large factor in determining the life span of a miner. Most people would agree that a miner becomes useless when it costs more to run than it can produce in coins. Therefore, as your power costs increase, your miner's lifespan will decrease.
Another factor to take into consideration is difficulty. Since difficulty changes are unknown, it's hard to determine how long a miner remains viable. Just like with power, the smaller the adjustments in difficulty, the longer your miner lives.
You can also factor in alternate coin mining. Sure, it may become unprofitable to keep your miner working on BTC, but it might make sense to mine a few other coins before sending the hardware to the recycling bin.
Of course, all of these answers are based on the assumption that your definition of a miner's lifespan is driven by profit. If profit is not a concern to you, and you simply want to mine coins to support the network, then your miner's lifespan is as long as the hardware functions.