When did content become a noun?

Solution 1:

According to Webster's Ninth New Collegiate Dictionary, the first documented use of "content" as a noun with the sense of "something contained" is in the 15th century.

Solution 2:

The OED has content as a noun as early as the 1500s:

• 1526 Pilgr. Perf. (W. de W. 1531) 6 b, ― All this worlde with the contentes in the same.
• 1547 Boorde Brev. Health lxxiii. 23 ― Yf in an urine doo appere a content lyke as heares were chopped in it.

However, your particular sense might be the one first shown in 1901:

• 1901 Chemist & Druggist LVIII. 18 ― Jeancard and Satie··conclude that altitude has no influence upon the ester content of lavender oil.

There are also earlier uses, but that looks to be the best match for what you mean.

• 1539 Bible (Great) title-p., ― The Byble in Englyshe, that is to saye the content of all the holy Scrypture, bothe of yᵉ olde and newe testament.
• 1594 Plat Jewell-ho., Diuers Chim. Concl. 28 ― A glasse··of some greater content.
• 1660 Barrow Euclid i. prop. 35 schol., ― The area or content of the Rectangle.
• 1863 Huxley Man’s Place Nat. ii. 77 ― The most capacious Gorilla skull yet measured has a content of not more than 34½ cubic inches.