Translation:He only has one jacket and he wears it all the time.
I agree that context should indicate, but a single sentence doesn't provide sufficient context. If the context is "I have a rain jacket and a windbreaker, ... " then "he only has one jacket" makes sense. But if the context is "I have a parka and a rain jacket and a sweater and a fleece" then "he only has a jacket" makes more sense.
After a year long session of seeing un, uno, una all the time, I just know naturally now. One of the nuances of learning a romantic language is them pesky definite and indefinite articles. From what I can gather is routine practice and memorization is the only way to get it right.
"interchangeable" is a pretty strong word
I think CheriStead has laid out pretty well the different contexts in which one or the other makes sense for the first half of this sentence, for example. Certainly either could be used in the sentence "He only has [...] jacket," but the foregoing context can determine which one fits.
Of course, there are also trivial examples: "He has two jackets. I only have one."
Both are correct grammatically, but only one is a correct translation of the sentence given. I would say these sentences are cousins, but not twins. "All the time" suggests he wears that jacket often, maybe every day. However, "every time" can be a lot more limited --- he wears the jacket every time it rains, or every time he goes to a concert.