Right click on any folder, Properties, Retention Policy.
Also read this:
https://support.mozilla.org/en-US/questions/1113291


Mike
 

On 5/2/20 2:58 AM, users-request@lists.fedoraproject.org wrote:
Sir, you have converted me into a Thunderbird user. It is awesome. Let me know if the quoting is correct or not.

I just have one question - my mailbox is huge but I want Thunderbird to keep mail for lets say the last 3 months. How do I do that ?

-- 
Regards,
Sreyan Chakravarty