The required information on the chip does not seem to be published. So you are in (officially) uncharted waters.
Many or most MCU ADCs have an input resistance that varies depending on how frequently you read them. There is also the leakage to contend with, but it's often a secondary concern.
A guess would be that if you keep the source impedance (R1||R2) less than 5K the accuracy will be little affected. Most microcontrollers in my experience are most accurate with between 2.5K and 10K maximum.
If you want to go higher (to reduce current) you can either perform tests to try to qualify the particular part you have under your expected conditions (a capacitor across R2 may help), or you could add a CMOS op-amp to buffer the voltage divider and go much higher in resistance (but you have to buy and power the op-amp). Or you could add a high-side switch to switch a low-resistance voltage divider 'on' just during measurements.