You're asking the wrong question: It should be, "How often do BA and DL run these?"

First, thanks for pointing out DL's extension to the 50% mileage bonus. DL has been doing this for a while, for a few months at least. And BA has been doing this for quite some time; however, the bonuses started out at 25-30%. During the first half of the year, the transfer bonuses were 40%.

Second, in my short time that I've been in the AMEX mileage factory business , the most I've seen in terms of transfer bonuses was 50%.