3500 water pump

2006 impalla 3.5, how many miles on the water pump before they leak?

Difficult question to answer , my 2000 Impala 3.8 L made it to 130,000 miles ( over a 10 year period ) before it started leaking ! Yours may be less , may be more ?

the car has 142000 and it looks stock.

Why are you concerned? Is there a rumor around about failures? I remember back when a water pump was about a 60K mile maintenance item, and exhaust systems would last about 3-4 years no matter what. Those were the old days. 100,000 hour bearings are very common any more. Poly-V belts (serpentine) use automatic tensioners so that accessory bearings don't get overloaded like they used to. Water pumps tend to erode less and the pump seals are lubricated better with the newer coolants. Still, if it's a weak point it might be a good preventive maintenance measure, especially if a leak will cause other damage (like the LT1s suffered).