Just did some day-to-day testing about research time in SimCity. If only SimCity would tell us more information about length of projects, or what is happening, but once again it's up to the community to figure it out. The research project time is indeed based on the amount of students attending. And yes, different research goes at different speeds. Here's what I found:
Clean Coal Generator:

6% completed from 722 students. 0.83% researched per 100 students in one game-time day. So if you averaged 1000 students per day you would get 8.3% completed every day. Or around 12 game days to complete it.
Clean Oil Generator:


10% completed from 1000 students. 1% researched per 100 students in one game-time day. 10-12 days for Clean Coal/Oil with 1000 students.
Concentrated Solar Array:

11% for 1476 students or 0.74% researched per 100 students in one game-time day. Or something like 13 days to complete.
Solar Farm Great Work:


9% for 1841 students or 0.48% researched per 100 students in one game-time day. Or something like 20-21 days to complete.
I'm not very convinced at the reliability of these "students taught today" numbers but it seems with 1000 students it would take 10-13 days for the non-great work projects and 20 days for the great work projects.