When is a good time to ask for a letter of reference from a professor?

Ideally, the letter of recommendation phase is incredibly beneficial because a) a higher level person is saying awesome things about you (rather than you bragging about yourself like you do elsewhere in the application  b) someone else does all the work for you (you just influence their perceptions of you).   In your case, let's get the standard answers out of the way (get an A, participate in class, go to office hours, be personable).  Six to twelve months after the class is over is ideal.  First of all, get an A or A-.  A B level student gets a B level of recommendation.  Next, send a series of emails to the professor at a frequency of every 2-3 months about how you used some topic in his/her class to your advantage (personalize something like I helped my parents qualify for a mortgage because I was able to reframe their assets and liabilities in a favorable manner after taking your accounting class) or say how you understood some national event better because you took the his/her class (i.e. the macroecononmics class you just took helped you understand "too big to fail" in a whole new light during the 2008 financial meltdown and how it made you rethink your investments).  DON'T give bland pleasantries like "I really enjoyed your class" or "this was one of the best classes I took in college."  Professors smell a lack of authenticity a mile away. FYI, you by emailing every 2-3 months, you allow the professor to remember you, forget about you, and remember you again (thus promoting you in their heads through spaced repetition. On your third post-class email, state that you would be "honored" to have this professor write a letter of recommendation on your behalf for graduate school.  Schedule the meeting ideally as the last appointment of the day so they can't bump you out if they have a meeting afterwards.   Before the meeting, do as much research (or pay a virtual assistant to do it) about the professor as possible (i.e. where they went to school, their social media pages, areas of interest, where they grew up, publications, fan of any team/TV show, etc.).  You don't bring up anything bluntly because they will see you are trying too hard (i.e. "Are you a fan of any sports teams?" or "I heard the TA say you are a big Simpsons fan"). However, if they say something casually-like "I just came back from a healthcare symposium from Japan last week" then you can say "Did you bring back any Mr. Sparkle?" (a true Simpsons fan would recognize the reference to an episode about how Japanese corporations used a logo that resembled Homer coincidentally). That builds rapport and points with the professor by doing some research in advance about the professor. Dress business style (people get impressed when you dress for them).  Bring professional ammunition with you so basically you give the letter of recommendation writer details to write (they have more important meetings/deadlines/presentations to do so the more you do the work in advance the better).  Bring some kind of folder or case to bring your materials. Always have a professional CV (have it edited, pay someone to reorganize it professionally with accurate citations, and use fancy think paper).  In addition, if  you have many publications/awards/recognitions, you can essentially go to a copy center and have them make a spiral-bound book about yourself.  Use dividers to separate your CV, publications, awards, recognitions, personal statement, anything of tangible value.  Don't bluntly present it to them.  At some point, they will ask questions like "Why do you want to be a graduate student" or "what type of psychology you want to be involved in" to get a feel for you.  You answer the question and then say something like "I've organized some information about my background for you to review. Would you like to see it" (no one says no). Then, you pull out your notebook (make sure it was out of sight before) and summarize the various parts of it. You look organized, you look credible, you look accomplished, and people like to walk away with something (plus no one had done this before probably). Of course, after you talk about yourself, you stroke their ego.   Tell them what you liked about their class (specifics) and how you are a better thinker/well-rounded person because of it.  By this point, they will eat up your compliments as long as you come across as sincere. If you are feeling very comfortable, have someone take a picture of the both of you.  Then, put it in your thank you note that you send at least two weeks after your meeting (because they will put it off until they are reminded if they are busy professors). In summary, after getting your A, you bombard the professors with a favorable image of you repeatedly in order to manipulate their mind before the letter is written.   Once you are regarded as credible and sincere, you are open to brag about yourself and compliment their teaching efforts without skepticism because the professor is malleable. They will be essentially programmed by you to write a great letter of recommendation for you.