The second stage is after the first shave or two (possibly 3, but not likely), when that marvellous initial edge has been dulled just a bit. It's still very sharp, but it's not quite as easy to cut yourself now, as the blade has been 'worked in', so to speak. This is my favorite stage.

The third stage comes when the blade is beginning to get dull, and the shaving gets harder. During this stage, you'll often find yourself pressing harder on the handle to get the same close shave you were getting at stage 2 without difficulty. This is another dangerous stage; using extra force on the handle increases the level of irritation, and increases the chance of nicks and cuts. When you realize you're in this stage, it's time to change blades.

How long each stage lasts depends on a number of conditions, which include (but are not limited to): the quality of the steel, the initial sharpness of the blade, the hardness of your hair.

For me, using typical Mach 3 blades, I tend to get 2 shaves at stage 1, maybe 10 shaves at stage 2 (at the outside). Then it's time to change blades. Your results may vary, so don't use my results thinking that they're any kind of benchmark.

How do you increase the life of your blade? There's a limited number of things you can do, but they will make a difference. Most importantly, soften your hair before you shave. That is best accomplished by wetting your hair well, and rubbing in a good shaving cream and letting it sit for a minute before you start. The water and shaving cream will soften the hair and make it easier to cut, which will extend the life of your blade.

There's a company called GreatRazors.com that cryogenically tempers razor blades, and claims that this can make a typical blade last 3 times longer than a non-cryo'd blade. I've been using a cryo'd Mach 3 for the past 5 weeks, and it's still going strong, so it seems to live up to its hype. Check out HeadShaver.org's personal favorites.

Source: Great Razors