Bedtime- crying

At what age did you stop going into your babys room to comfort them from crying after being laid down in their own bed?

At what age did you start teaching your baby /babies to self soothe for bedtime when they are crying?

ADDED: I guess I should add that our daughter is 9 months old and has been an AMAZING sleeper since we brought her home as a newborn. We are first time parents so this is totally new for us. Since she has learned how to crawl, she has been waking up an hour or so after I lay her in her bed. She has been sleeping in her own bed in her own room for 3 months now without any issues until maybe 2 weeks ago. I guess I'm just not sure what is appropriate or not 🤷‍♀️