The best length for practice
Three seconds is short enough to click flat out but long enough to show whether a jitter or butterfly rhythm holds together. Do five attempts with short rests and watch the graph for dips rather than chasing the score.
What a 3 second score tells you
Most people score around 7 to 8.5 CPS, about 21 to 25 clicks. Around 27 clicks is fast, and 40 or more usually takes a clicking technique.