So, I searched around a bit and didn't find anything specifically about how long a set of SPD cleats might be expected to last. I'm using a MTB shoe and don't walk on them very much. I'm currently riding about 36 miles a day on my commute. The cleat itself seems fairly soft and I think would wear pretty fast against the clipless pedal components which seem to be quite hard.

Any input??

I have never worn a pair out. I have one pair that must be ~20k miles along now. On its 4th or 5th pair of shoes. I can't believe the shoes fall apart like they do!!

Walking is what kills them.

the pair I use on my commuter shoes only last about a year because I walk on them. They are installed on mtn bike shoes, but still hit the ground enough to wear them down after a year.

That being said, the pair on my spd sandals are 8 years old and still work as good as new.

it depends on the brand, Shimano SPD are steel, and last forever. I've worn out pedals on one pair of cleats. Crank Brothers on the other hand use a brass cleat that wears out much quicker. Then again, the pedals wear less, so it all works out to be more or less even.