Type this in the terminal:
defaults write com.apple.dt.Xcode ShowBuildOperationDuration YES
Duration appears in the activity viewer after a build, alongside the “Succeeded” message.
If you are running the app, the status will be replaced by the running status before you can see the duration.
This replaces the entry that was used in older versions of Xcode:
defaults write com.apple.Xcode ShowBuildOperationDuration YES
Xcode may need to be closed before you enter this command. Durations should appear at the bottom left of the project window.
Comment from an Xcode developer: “As with all undocumented user defaults, this is unsupported, assumed (but not guaranteed) accurate, and not assured to be effective in future versions.”