how long I have to wait before writing the next command to the AD5671R?
The time between writes will be approximately the sum of frame time plus the settling time.
Please note that the settling time is a factor of the slew rate (~0.8V/uS), voltage change and the load capacitance.
You may refer to Figure 48 of the datasheet for the typical settling time for various load capacitance.
Retrieving data ...