According to me, it is not possible to determine the result of an email in 24 hrs. One month will be an ideal duration for an understandable test.
When writing an email, we should consider the product or service. Then about its nature, segment, season, political situation, and all other factors that can affect the results. We should pick general conditions. Also should implement two versions with some changes. to each other. After this, run the test and analyze CTR after a month.