How long can use a razor?

Ok so I'm 14 and I have not asked to shave my legs yet, but my brother turned 18 and Gillette sent my brother this free razor and he never even used it, it was just in the box and so I took it and started shaving my legs, and I did that in December and I use it like 1 or 2 times a month because I'm afraid the blades are going to become dull. The razor still works and it shaved my legs fine. How long can you usually use one of these?