The best time to revise your research questions is when you discover that there is very little information about your topic.
The reason for this is that if you discover that there isn't a lot of information on the topic you want to investigate, then this may mean that you won't be able to conduct any reasearch on that. So, before starting a project on something you're not sure it can be done, you should revise your research questions and check if anyone else has written anything about the topic you want to investigate, in other words, check the existing bibliography.