I was told that you have to buy the same brand of replacement blades as the cutter. So if it's an Olfa, you'd need Olfa blades - for Fiscars, it would be Fiscars blades. Supposedly the hole in the middle is different and they are not interchangeable. Since I only have one type, I never tested the theory.
How long they last depends on how carefully you use it and how stubborn you are.
1. When you use the cutter, make sure you have nothing but fabric on the mat. Don't run over needles for example.
2. Make sure to close the blade after every use. That protects your fingers and the blade from getting nicked.
3. When you make your first cut, don't start at the corner of the ruler. Instead, start a little ways inside the ruler and pull the blade back to start and then push forward.
4. Make sure you hold the blade at a 90 degree angle tightly against the ruler.
5. You can tell that your blade is getting dull when you have to apply more pressure to cut through the fabric or when you have parts of the cut that are not completed. If you are stubborn, you will fight this for a while and hack at the fabric. (Yes, I am somewhat stubborn). It doesn''t really pay off. You spend more time being frustrated and putting more pressure on your wrist.
6. There are blade sharpeners on the market. I have a manual one but it really doesn't buy the blade that much more life. None, if the blade is nicked.
7. I buy them on sale at Joann or with a coupon. Whatever is the best deal and what your wallet will allow.