the 3 months ladder is associated with the hemoglobin lifespan, from 2 to 3 months. you therefore can monitor the HBA1c in lower time but you should really set the times in your protocol.
Ayala JE, Samuel VT, Morton GJ, et al. Standard operating procedures for describing and performing metabolic tests of glucose homeostasis in mice. Disease Models & Mechanisms. 2010;3(9-10):525-534. doi:10.1242/dmm.006239.