Never believe what garages tell you!!
If you can't check them yourself get a free check from an independent, and have a look for yourself. It seems that Nissan tell most of their customers that "your pads are 80% worn, and need replacing very soon".
They told me exactly the same thing when i took my car in for a service, so i went out and bought new pads and front discs, (performance ones, not oem) but didn't fit them for another 5000 miles. When i took the old pads off they still had 50% life in them. still changed them though!!