Should arrive around about the 15th or so. From then you can just go online and re-tax anyway as long as you've got the V5, you don't need the reminder.
Mine got here on the 8th of the month when mine was due..... I guess it's down to the mail and or Holiday times....but as others say....usually by 15th of the Month...You can renew your RFL earlier now than you used to be able to