Should I Wait to Buy My Next iPhone?

Back in the summer of 2011, I bought an iPhone 4. Yup, that's right, just three months before Apple unveiled iPhone 4S and Siri, I plunked down $200 for yesterday's technology. In my defense, I was changing jobs and needed a phone ASAP. iPhone was my first choice. Fast-forward to today and I'm now eligible for an iPhone 5 upgrade. That's good news, since I've tested the iPhone 5 and am impressed. So why don't I just run out now and get it? You know why. I keep wondering, how soon before the next...