When can I withdraw 400K from my Thai bank account after getting a retirement visa approval?

You can withdraw 400K baht from your Thai bank account 3 months after the extension of stay stamp dated 6/6/23, meaning you would be eligible to withdraw on September 7, 2023. However, some offices may require you to maintain the full amount until closer to your next extension, so it's advisable to check with your immigration office for clarification.