I measure FPG and A1c every three to six months. A prediabetic should also regularly test every 1 to 2 years for easily testable complications: eye exam; eGFR for kidney function. If you have money, you can add noninvasive artery scans for atherosclerosis progression.
Last year, I had had A1c's 6 months apart, both 37, down from 44. A sudden substantial weight loss made me test again. From 37 to 43 in 2 months. This upset me, especially since I thought I had been maintaining the new good habits. I asked for another test after only 2 months; up again to 44. Mysteriously, it's now 41.
In the last 3 years, the A1c has been quite up and down, the fasting rather steady. Measure both, then.
I don't get any of the major complications or the minor ones I'm reading about, like thrush. If they happen, I'll rush to get new FPG and A1c.