Keeping things VERY general, In terms of academic impact:
Prospective Double-Blind Randomized Controlled Trial > Prospective Randomized Controlled Trial >> Prospective Matched Trial (Both Retro and Prospective)
Systematic Review with Meta-analysis > Systematic Review
Retrospective Cohort Study>>>>> Case Series (no stats, just characterizing things) >>>>>>>>>>Case Reports
Regarding time, for prospective studies I've found the things taking the most time are IRB approval (my hospital takes forever), and patient recruitment. For retrospective studies, the amount of time needed depends on the number of patients (charts) to review and the number of variables in question.
Before doing ANY research, do a power analysis and a literature review. No sense in wasting your time if the topic is published ad nauseam or the number of patients needed to find a significant result is >10,000.
Don't waste your time on case reports.
@Newyear , feel free to PM me if you have questions. Currently on a clinical research year in residency.