Research is never enough! But based on my observation, some students tend to do research for the sake of research. For example, when I ask them to compile moodboards, they will go to Google Images, enter the keyword (e.g. 'ninja') and proceed to dump the first 10 images on the search results into their moodboard. My general advise is to do sufficient research to generate your first batch of silhouettes/thumbnail. Then assess your work (or get feedback from your teacher/classmates) and decide if you should proceed to the next phase or to do another round. Whichever is the case, you may need to go back to the research phase if you need to figure out something (e.g. you are not too sure how this particular detail on the character's accessory should look like). Sometimes the workflow does not move linearly.

Hi Mitsuki, yes I used to teach at the Higher Nitec in Games Art & Design course. Say hi to your lecturers for me ;)