Life expectancy depends on driving style and load. Lots of highway miles and you should be able to go well past 50k.
The only way you can know if you need pads is pull the tire and wheel, spray brake cleaner through the hole in the top of the caliper. Most pads have a vertical slot cut through fom top to bottom. If you can see the slot (light required) you have at least another 10k. Look carefully for even wear on the inside versus the outside pad.
Spend the money and buy a premium HD set of pads from a reputable vendor. I like Raybestos, but you have to be careful as they have several different grades of pads.