Once you get the medical request letter, you have 30 days to do it. I can't tell you when they will send it, however, you'll just have to be patient. I would advise you check your application status using ECAS. It's possible that they have sent the request and you have not received it yet (it would go to your GCKey account if you've linked your application, or would be mailed to you), and if that's the case then ECAS would tell you if they've sent the medical request