How often should I change my mountain SPD bikeshoe cleats?
I have similar problems as you describe, but mine come from dirt, mud, and other things that get caught in while doing MTB.

Aside from that, the Shimano SPD pedals develop some sort of a lip as they wear out, in the side "ramps" that rise the cleat when you twist the foot sideways. It happens in similar fashion as if you repeatedly hit the end of a metal rod with a hammer, until it deforms and starts to look like a nail head. This "lip" makes it very difficult to twist the foot, hence it is difficult to release the cleat from the pedal. The solution I have implemented is to file those lips out every now and then.

Close inspection to the pedal clamp mechanism is all it takes, scratch the ramps with your fingernail and if you feel a little stop near the end of it, then you file it and repeat the fingernail test until your nail can freely slide out of the ramp... Sometimes it is so little that even sandpaper can do the job. After that, clean the mechanism the best you can with a toothbrush and apply some lubricant. Apply lubricant also when you feel it's necessary. (I do it before every MTB ride, but almost every 10 rides for city commuting)

Regarding cleats, at least for me, I never change them, my shoes usually die first, then I try to put the same cleat on new shoes, and only if the bolt is too hard to remove (happened once) then I use new cleats...

My oldest pedals are 13 years old now, in the bike I use the most (the MTB for XC and light DH), have other 2 bikes with old SPD on them (commuting or urban riding), but only one pair of shoes for everything.