When should one ask for letters recommendations of the people of one's choice? E.g. when applying for a PhD program, should you ask for two or three letters all at once or should one collect a letter immediately after one has finished something significant like an undergraduate thesis, an (under)graduate research paper or Master's thesis, so that one has all the letters together covering maybe 2 or 3 years when applying for a PhD position?
You can choose either of the options, but I would wait at least till I've had quite a few accomplishment. However, some professors take a long time to write, so it would be good to approach them at least a few months before you start applying. Letter of recommendation can be collected and filed till the time you need them.