Is it 6 months from the date the medicals were taken or the date the results are updated by IRCC.There is no source for this info, but can be evidently noticed with the trend of applicants receiving re-medicals requests last week.
So, you can say, anybody whose medicals are expired, if you add additional "6 months" to it and if still it doesn't cross today's date, then most likely you'll 100% receive the re-medicals request.
This is what have been noticed, and basis this people have received re-medicals request or will receive it soon.
eg: Medicals taken 01st May, status updated 15th June by IRCC. From which date 6 months extension.