http://www.wisebread.com/By Silicon Valley ...It may be a little bit early to think about retirement all too seriously if you still have several decades left before you face it. But don't look now -- time flies and before you know it, there won't be much time to prepare for retirement. On this subject, the big question I have in mind is this: how do we know if we've saved enough for our retirement years? In order to maintain the lifestyle that we are used to, the conventional wisdom tells us th