Is there a consensus on how long you should know a person before you ask him/ her to write you a recommendation letter for graduate school? I was wondering if, in general, the length of time you have spent with your professor/ superior has a great impact on your application.
What I personally have done is that if I get an A+/A/A- with his course I would reach out to him for a recommendation letter. This might not be the case for several Professors (it is a case by case basis), but in general this works. I also would ask for recommendations for Professor's that I have done research projects with.