The longer your commute, the more self-sufficient you need to be. If you are only going 1 mile, you can always walk if something goes wrong. If you are going >10 miles, you need to at least be able to change a flat by yourself, be able to call someone for help, or have an alternative (like public transportation stops) that you know will be available.

I commute 23 miles round trip (11.5 miles each way). I wouldn't do it without the following: flat repair kit, tire levers, pump, spare tube, bike lock, headlight, taillight, multitool, fenders, a cell phone, and some type of bag to carry it all in.