For what it's worth, while I have graduated past the speedlight era and moved into studio strobes and on location lighting, the flash tube, much like any other flashtube whether or not its strobe or speedlight, has a rating of 250,000+ flashes... Granted, at the 250K mark I can replace my flashtubes myself where the canon speedlights, you cannot, but it is what it is... The 5,000 flash rating the strobist link mentioned, assuming you take his word on the odometer thing, has many variables attached including as he said how often you fire 1:1, recycle time inbetween fires, physical heat and humidity, etc... also keep in mind that if there is any leak or any debris, even dust get into the housing and get stuck to the tube, that could shorten it's lifespan much like if you were to touch the flash tube with your dirty oily hands. Also if you are shooting the hell out of it in the middle of summer with 100% humidity and not letting it cool down, you can expect it to fail more often than not. Lastly, if your being kind with your flashes... (bumping up the ISO to compensate for lower flash power, giving it time to cool down, alternating between flashes if needed, etc), then you can expect them to last quite a bit longer. Plus, Canon replaces their flash units every few years, so by the time you likely would kill your flash, there would likely be a new and better "toy" to play with rather than paying for a new flashtube.