The AHRS filters are normally not called at a fixed frequency, but on each new IMU measurement. Hence it depends on your IMU frequency.
For some AHRS filters you can also set AHRS_PROPAGATE_FREQUENCY to use a fixed dt value instead of the time between two gyro measurements.
The (usually not recommended) euler complementary filter is also called on each new gyro measurement (see gyro_cb() in ahrs_int_cmpl_euler_wrapper.c), but the implementation itself assumed this is 512Hz (as defined in F_UPDATE).
So to sum up: In most cases the AHRS freq depends on the frequency your IMU driver publishes new measurements via ABI