How often does the 4046E complete a full RSSI frequency sweep?

The answer is, "it depends". 

The sensor sweeps the full defined range in 25 kHz steps at ~10 ms/point, along with a hard-coded 10 ms between points. 

While this does seem quite fast, consider the worst case scenario where the 140 MHz span is requested. 

140,000,000 / 25,000 = 5600 samples

5600 samples x (0.01 s + 0.01 s)  = ~112 s = ~1.87 min

Another example: The RSSI sweep is defined between 450 and 485 MHz:

35 MHz / 25 kHz = 1400 samples

1400 samples * 0.02 s = ~28 s = ~0.47 min

Another example: The RSSI sweep range is defined between 880 and 905 MHz:

25 MHz / 25 kHz = 1000 samples

1000 samples * 0.02 s = ~20 s = ~ 0.33 min