We used the Thunderbird18.104.22.168 release to get Rick Tessner at MozillaMessagingCo up to speed. There’s a lot of Build mechanics to take in, so its not fair to add extra pressure by measuring all the wall-clock times.
Rick is also working to have the existing release automation we use for Firefox be used for Thunderbird also. In theory, it should just work, and initial experiments seem promising, but we’ll need a full test cycle on this before we can switch over in production. The curious can follow along in bug#427769.