So, when should you actually put in a circuit breaker?

hey everyone, i'm trying to get a better handle on when exactly it makes sense to implement circuit breaker patterns in our microservices. we've got a few critical services, and i'm worried about cascading failures if one of them hiccups. are there specific metrics or failure rates you look at? or is it more about the criticality of the downstream service? curious to hear your experiences and rules of thumb here