I will try to answer some of the questions. Firstly, it can take up to 120 days after the request when they get back to you eventually... It all depends on many factors, like your officer, medical (when were submitted and any additional tests are required). Secondly, the DM can come either by email, or mail. I received mine by email, however everything else was by mail...and they used my old email address, which is marked "expired" on my GCSM notes...weird...Lastly, DM letter will not have information regarding the date of your interview, only the landing office. The landing letter will come later and the waiting time depends o the location of the landing centre.