I personally use the Gillette Mach 3 razor but I have a problem. The blade doesn't stay sharp long enough it seems so I was wondering how many times should I shave using the same blade before replacing it?The question of blade life is highly dependent on the individual. Some people have soft, sparse hair and can use a single blade for quite a while before it gets dull. Some people have very thick, coarse hair and they go through blades quite quickly. As a result, there's no way to be sure how long a blade will last for you.
Blades should be changed when they start to get dull enough that you find yourself using more and more pressure in order to get the same quality of shave. This is often noticeable when you start to feel more irritation when you shave, because irritation is often caused by pressing too hard with the razor while shaving.
As a personal rule of thumb, I tend to keep an eye on the 'comfort strip' on my Mach 3. When the comfort strip starts to crumble and disappear, that's when I consider changing the blade. Note, that's not the same as changing it when the color fades from the strip; that usually happens pretty quickly, but it's not a good indicator of blade life (as we discussed in this article). Even when the comfort strip is mostly gone, the blade might be sharp enough to use, but you have to develop a bit of a feel for that.
There are ways to increase blade life. Ensuring that your hair is as soft as possible before shaving (by making sure it's very wet, like shaving during or after a shower) helps. Storing the blade in mineral oil or something similar helps cut back on corrosion on the blade; there are commercial products like Edge Saver that do the same thing.
Cryogenically-hardened razor blades are also available, which tend to last longer than normal blades.