Whilst haemoglobin lives about 3 months, it doesn't all die at once, so at any given time we have older and newer cells in our blood.
Whilst the NHS does tests every three months, the result of an A1c most closely reflects the condition of your blood over the last couple of months, so redoing every couple of months can be useful.
I know of a GP who works with many patients living with T2, and for those adopting a LC diet, he retests at 6 weeks, as that gives an indication of what's going on, and his patients find it very motivational.
If your GP won't agree to an early A1c test, you could do something yourself. You could use a private testing service, such as MonitorMyHealth (there are others out there too, like Medichecks, Blue Horizon), who run an A1c, from a finger prick test for £29
https://monitormyhealth.org.uk/ For this you collect your blood sample at home and post it off, with results arriving within a day or so of the sample reaching them.
Or you could do one at home, again from a finger prick test. I have used these tests:
https://millermedicalsupplies.com/d...g/hba1c/a1cnowr-hba1c-whole-blood-tester.html Although that particular pack contains 10 tests, you can buy packs of 4 tests, but the saving isn't proportional.
With the A1cNow tests, you do those totally in your own home (or wherever you choose to do it), so no post involved.