Inventory Turn Ratio

I was reading the book Financial Statements by Thomas R. Ittelson. I had a basic doubt on the calculation of Inventory Turn which is the ratio of COGS (from the Income statement) and inventory (from the balance sheet).

While the income statement reflects the picture in a period and balance sheet at a current point in time, how can we compare the same? Or in other words, COGS will continue to increase as time progresses, but inventory depends on holding capacity and the business type, will not the ratio also be a function of time the business is running?

I am not from an accounting background, so apologies if this is very basic or covered elsewhere. Links to further reading would be helpful too. 🙂