code: public ServiceCachingPolicyBuilder withMaxServiceInstanceIdleTime(int maxServiceInstanceIdleTime, TimeUnit unit) {
          checkState(maxServiceInstanceIdleTime > 0);
          checkNotNull(unit);
  
          _maxServiceInstanceIdleTimeNanos = unit.toNanos(maxServiceInstanceIdleTime);
          return this;
      }
docs: Set the amount of time a cached instance is allowed to sit idle in the cache before being eligible for
  expiration.  If never called, cached instances will not expire solely due to idle time.
  
  @param maxServiceInstanceIdleTime The time an instance may be idle before allowed to expire.
  @param unit                       The unit of time the {@code maxServiceInstanceIdleTime} is in.
  @return this