You should aim to be as extensive as possible when conducting searches for systematic reviews. However, it may be necessary to strike a balance between the sensitivity and precision of your search.
Sensitivity – the number of relevant results identified divided by the total number of relevant results in existence
Precision – the number of relevant results identified divided by the total number of results identified.
Increasing the comprehensiveness of a search will reduce its precision and will retrieve more non-relevant results. However,
… at a conservatively-estimated reading rate of two abstracts per minute, the results of a database search can be ‘scanread’ at the rate of 120 per hour (or approximately 1000 over an 8-hour period), so the high yield and low precision associated with systematic review searching is not as daunting as it might at first appear in comparison with the total time to be invested in the review. (Cochrane Handbook for Systematic Reviews of Interventions 2008, p. 130)