No, the noun phrase “15 days sick leave benefit” in that one-sentence paragraph isn’t grammatically correct because it isn’t properly punctuated. As it is, that noun phrase looks and sounds as if its words were just thrown helter-skelter into that sentence, making its sense difficult to figure out.
There are four options to make that noun phrase right:
(1) Write it as “faculty sick-leave benefit of 15 days” so there’s no doubt whatsoever which word in that noun phrase is modifying which, thus making the entire sentence much easier to understand: “In terms of benefits, Cao mentioned the faculty sick-leave benefit of 15 days that may be monetized upon retirement...”
(2) Write it as “15 days of faculty sick-leave benefit,” with the preposition “of” inserted between “15 days” and “faculty sick-leave benefit” to make it clear that “15 days” is a quantifier of the noun phrase “faculty sick-leave benefit”: “In terms of benefits, Cao mentioned the 15 days of faculty sick-leave benefit that may be monetized upon retirement...”
(3) Write it as “15 days’ faculty sick-leave benefit” with an apostrophe after “days,” so that it becomes clear that “faulty sick-leave benefit” is the subject noun phrase and that it’s modified by “15 days” as a quantifier: “In terms of benefits, Cao mentioned the 15 days’ faculty sick-leave benefit that may be monetized upon retirement...”
(4) Write it as “15-day faculty sick-leave benefit” with the “s” in “days” dropped and a hyphen placed between it and the number “15,” so that—as in Option 3 above— it becomes clear that “faculty sick-leave benefit” is the subject noun phrase and that it’s modified by the compound adjective “15-day” as quantifier: “In terms of benefits, Cao mentioned the 15-day faculty sick-leave benefit that may be monetized upon retirement...”
Take your pick. Stylistically, though, I prefer Option 1 because it’s clearest, neatest, and simplest to execute, followed by Option 2, which has the virtue of emphasizing the value of the benefit being described. The other options require a lot of judgmental grammar decisions that might just confuse both writer and reader.
