About two months ago, I started using dlvr.it to cross-post shortened links from this blog to Twitter. I soon discovered that I could use it to track Twitter's impact on my blog traffic in real time. To my initial surprise (but makes more sense on reflection), some 4-5% of my "followers" click on the link within a few minutes of posting. It's pretty well peaked out within an hour or so at a median of 7 or 8% or "followers." Top performers get to around 15% of "followers," but in most cases that's because of retweeting, which you can detect because traffic builds for a longer period.
Tweets, therefore, have a very short useful lifespan. I tend to do blog posting late in the evening, but it makes sense, if you're using Twitter to build blog traffic, to save these and post during higher periods of Twitter usage. --Dennis