While a well-planned narrative review may take weeks to months to complete, scoping reviews, systematic reviews, meta-analyses, and other related review types can take considerably longer.
Systematic reviews should not be used as instructional lessons or short summer assignments unless ample time can be dedicated by a team beyond the course, internship, fellowship, or other training program.
Systematic reviews typically take a year or more to complete, though this may vary depending on the size of the team and the scope of the research question. Here is an estimate from Cochrane, an organization behind what is considered the gold standard of systematic reviews:
The PredictTER tool may help you estimate the time needed to complete your systematic or related review type.