It will all depend on your riding style. My last set of disc pads lasted about 1 1/2 months. My set before that lasted about 8 months. I'm riding much more aggressive now though and a lot of the riding has been in poor conditions.
Keep in mind that pads usually cost around $25 a set (so $50 for front and back).
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I may be fat but I'm slow enough to make up for it.