I strongly suspect the 6 monthly is more than adequate – as to how quickly it can go bad, I guess that’s all down to your control.
Mine has never been (anywhere near) perfect, I’m the first to admit that. I thought my last hba1c of 9 was quite good (for me!)
Just as I’m writing this I’ve had the call : Surgery booked for 3rd December (could of had next week but I’m not mentally ready yet!)