Usually 5 to 6 weeks before it is due.
Was the car originally registered in Britain by any chance? If so you won't get a reminder as it's under the DVLA system and MOTs here are handled by the DVA NI system. Once it has one MOT you will get reminders from next year on.
Got a reminder the other day for end of jan
Thought you might have needed the letter reference but I think I'm thinking of the VED reminder instead.