whens the right time to introduce a feature flag, really?

hey folks, i've been thinking a lot about feature flagging recently. we're a growing microservices shop, and deploying new features without impacting existing users is becoming critical. i've seen teams implement feature flags very early, sometimes for minor ui tweaks, and others wait until a feature is almost complete. what's your experience? when do you find feature flags provide the most value, and what are the trade-offs of introducing them too early or too late in the development cycle