I'm interpreting what you've written as follows: Raw product is accumulated. Finished product leaves as soon as it is created, and PL is the rate of conversion from raw product to finished product.

At time t = 0, there is some amount I of raw product accumulated. The finished product starts getting made at rate PL.

At time t = 3, raw product has run out yet (I > 3 * PL), and new raw product starts arriving at rate PA > PL.

At what time does the quantity of raw product return to the initial level I? The deficit created was 3 * PL, and the rate of growth is now (PA - PL). So the additional time required is 3 * PL / (PA - PL).

Cheers, Wayne

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