Hemoglobin A1c (HbA1c) is a long-term measure for glucose concentration in plasma. Since its introduction as a diabetes monitoring tool, and its more recent application as a diagnostic tool, the number of measurements of HbA1c have risen dramatically. However, HbA1c change is slow, so repeating measurements should not be done too often. We use a large, unfiltered dataset from 52,017 patients to determine the possible rate of change in HbA1c concentration. In our laboratory, the critical difference between HbA1c measurements is 8.5%. Our data show that a 1-unit HbA1c rise takes 4 weeks to occur, hence, at a HbA1c concentration around 50 mmol/mol Hgb, a critically increased HbA1c concentration cannot be determined until after 16 weeks. Conversely a critically lower HbA1c can manifest itself after 2 weeks, but after 7 weeks the dropping tendency stops. The amount of measurements that can be cancelled because they were taken sooner than 16 weeks is 23 percent.