Brake pads and discs are checked during an MOT and you can expect to have to replace your discs every 40 – 50 thousand miles and you discs every 60-70 thousand miles on average. Although pads and discs are not a huge expensive a full set all round will often cost a few hundred pounds plus and therefore the longer you can keep your brakes maintained the better.
Although your front brake discs are used a lot more than your rear brakes, you may find that the rear ones have to be replaced more often and this is usually due to corrosion. Discs and pads are made from cast iron which is an ideal material to use but the down side is it rusts. The front brakes can quickly wear off any rust that has accumulated by normal braking on a journey whereas the back ones may not be used for a while and therefore the rust just builds up. This can cause pitting on the brake discs and can weaken them meaning they will need to be replaced. Sometimes with a bit of heavy braking, you can remove some of the rust off the discs but if it has built up too much for example, if it has been sat for a long time or over driven short distances, then you will have to fork out for a new set.