IME you get an email inviting you to either keep or gift the MQMs a day or so after the closing date of the billing cycle in which the $30,000 or $60,000 spend is reached. Around the same time, your 15,000 bonus RDMs appear in your account but you only get the MQM automatically about three months later if you fail to make the selection.

At least once I did the selection before receiving the email link, IIRC by either searching on delta.dumb or using a link posted here by someone. It might have been the case that the RDMs had literally just appeared in my account.