Most thorough reviews require at least 6 months to complete, and often take 1-2 years. The resources below provide a sense of what to expect for a systematic review. but are applicable to most comprehensive reviews that follow established methodology. 

Source: Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from https://training.cochrane.org/handbook.

A study which measured time requirements for systematic reviews not published with Cochrane found that the average time from start to finish is 1 year. The study below includes analysis of major review tasks. 

Borah R, Brown AW, Capers PL, Kaiser KA. Analysis of the time and workers needed to conduct systematic reviews of medical interventions using data from the PROSPERO registry. BMJ open. 2017;7(2):e012545. View full text