How long should brake pads last?
On Rim Brakes:

I'd be surprised if you managed to wreck a set of brake pads in a week (or even several hundred miles in nasty commuting weather). Off road riding (MTB, cyclocross, etc.) can chew through a set of pads within a ride or two though depending on the terrain and the technicality of the ride. If they're trying to sell you new pads on a bike (instead of possibly adjusting them), I'd be very suspicious.

You can tell if you need new pads by either (a) the pads have been on there for a few years and the rubber has hardened up and they aren't working or (b) if its due to wear, 



new pads look like the top , old pads look like the bottom [the grooves should be nearly gone] (Image from here). 

[The bottom pads have a lip on them, which indicates that they weren't aligned properly to begin with. If your pads have just developed a lip, you can cut it off and use the pad as normal after re-aligning the pad.]

If they want to replace the whole brake, then there may be something wrong with the brake that you haven't mentioned (bent brake arm, or something).

On Disc Brakes: 
If the bike has disc brakes, one reason why the pads may need to be replaced is if they have gotten oil on them, in which case they do need to be replaced (and the rotors cleaned).

In all cases:
You should really go back and ask them why they want to replace the brakes/brake pads to begin with. Pictures of the situation would help too.