This is one of those questions that can be found through a close reading of the text. Faulkner does make it difficult to track time though, so it is a bit tricky. He doesn't write the story sequentially, so it is hard to figure out exactly how old she is in each section. But, there is only one place in the story where Faulkner mentions Emily doing anything to earn a bit of extra money. If you look at the text, after the story mentions Homer disappearing forever, almost near the end, Faulkner writes:
"From that time on her front door remained closed, save for a period of six orseven years, when she was about forty, during which she gave lessons in china painting."
So, at age 40, for about 6 or 7 years, she gave lessons on painting china. It is an interesting thing to do to earn money, and even more interesting that Miss Emily, from wealthy relations and a stubborn sense of pride, would condescend to teaching painting lessons for money. But, she does it. I hope that helps!