It depends on what you’re trying to measure.
You can profile a specific component’s render() method by putting the start() / stop() in componentWillUpdate / componentDidUpdate.
This also applies to your top-level component if you want to profile your whole app.
You can also profile certain interactions by manually calling Perf.start() in the browser developer console, performing the action, then manually calling Perf.stop().
There are some helpful links with more info here: https://facebook.github.io/react/docs/perf.html