Timing is crucial when requesting a letter of recommendation (LOR) from a professor. It's generally advisable to ask for a letter when preparing to apply for graduate school, rather than immediately after completing a course. This approach allows for a more personalized and detailed recommendation, which is more valuable than generic statements about performance. If a student has already secured strong LORs from professors with whom they conducted research, a third letter from a course instructor can still be beneficial, but its impact may depend on the quality and substance of the letter compared to competitors' applications. It is recommended to inform the professor at the end of the term about the intention to apply for graduate school, which can help in securing a more meaningful recommendation later.