It's hard to say because it depends on the field. For example, 50% of our customer names are over 50 characters, so that's a common issue.
If there were no other option, I'd say 100 would alleviate most issues, however, as you have a "Max Length" constraint on Field Preferences, what about a "Max Search Length" constraint (defaulted at 50)? (I know...sure, I can just pop that in. :) )