I may suspect you have not gotten told what half-time is? :-)

Halftime is when half of the product is gone. Let's say you have 100mg after 10 minutes is only 50mg still active, again after 10 minutes is 25mg still active, 10 min more 12.5 then +10 minutes 6.25mg, now 10 min later 3.12mg and that keeps going on.

For every 10 minutes is the product's strength half. It's whole time depends on when, what is left, is too little to do anything.