I’ve been using the same Speedplay pedals and cleats for a couple of years now. Lots and lots of miles on them. They look beat to hell but still work…easy to engage and release but still stay in nice and tight when jamming hard.
How can you tell when they are worn out?
When should you replace them?
I replace mine when I see signs of significant wear to the retention spring (the metal half circle). If you are getting a few years out of yours you need to ride more! 
I find that mine are harder to engage when the get old. So, when it’s hard to click in, I replace them. Usually every 3 years. But I keep the old ones on my winter shoes. I’m guessing that I get 12 to 15 thousand miles of outdoor riding on them, then maybe another 5 to 8 indoors.