How long to idle before entering sleep mode is entirely up to your usage pattern and desire. Do you want to be able to stumble off to bed without worrying about the computer being on forever? Do you tend to use the computer to read things that you then sit and ponder, occasionally rereading a passage or two? That's what I mean by usage pattern.
As to the ability of sleep mode to extend the life of your computer, I haven't seen any studies suggesting that it has much effect. Of course, I haven't looked for any. The main argument for its use is the cost savings and environmental benefits associated with using less power.
As to the charge that companies are making crappy computers, I'm not sure that is any more or less the case than it ever was. I think that companies like Dell build their computers based upon what studies show the average life-cycle to be. If the average is X then they build their computers to last somewhat longer than X, since that will suffice for the majority of their customers. Of course, the rate at which operating systems outgrow the ability of older computers to handle is another factor, so think about how often you upgrade your OS.
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