Let's make it nice and simple:
We know that the average exhalation is twice as long as the inhalation, which gives us a ratio of 2:1 (exhale:inhale). Out of every 3 seconds, exactly one is spent on inhalation.
We also need to know the frequency the respiration per minute.
Now, let's make good use of all this information:
Inspiratory time (s) = (60 s / Frequency) / 3
Inspiratory time (s) = 20 s / Frequency
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