The maximum sampling rate for current profiling (ADCP) or bottom tacking (BT) depends on a variety of factors, including the configured BT range and the use of an altimeter(ALT). 

If you are operating the instrument at a depth of 50m, the maximum sampling rate achievable is 4Hz. This ensures that the BT pings separates each ALT and ADCP ping, as illustrated below:

Answer: at max range, 1.3Hz

If the instrument is operated within a limited range of 25m, a higher sampling rate of up to 8Hz can be achieved. 

Answer: 2.6Hz

If the instrument is still operated at 8Hz, but you change the ALT sampling rate to 2, the sequence would change to:

The reason for this is that the unit doesn't define specific "seconds" but operates continuously, resulting in an irregular sample rate.