If you do not store your razors in the shower while not in use, the razors usually last about 2 months. Each cartridge pack comes with at least 4 razor blades. Definitely worth the extra money. They last long and do the best job at getting and keeping that smooth feel.
If you clean it and dry it and store it OUT of the shower, the blades can last for months! Proper care makes them stay in good, working order for a long time so that you can get your money's worth out of them!
If you take care of your blades they will take care of you. Apply a shave gel/foam or even hair conditioner on your legs before shaving. Rinse and dry your shaver and store in the bathroom cabinet. Shave regularly so the hair is shorter and thinner putting less wear on the blades. You will find you will keep the same one for up to 2 months.
I can make mine last about a month, sometimes even two. Keep it in a dry place, wipe it off when you're done with it, don't shave every single day and it should last you a long while before having to replace it.
Each razor lasts about a week. Depending how often you shave. But a box of them usually comes with 3 at the least I believe. These are my favorite razors because they don't give me razor burn and they work very well.