You can get almost anywhere within half an hour tops, the Metro runs till after midnight weekdays and 24/7 on the weekends, there's an extensive system of Tramways & Busses to cover anything a bit farther from a Metro station.
And you can get a yearly subscription, giving you all-inclusive usage of the services for 1€/day, billed monthly.
This is mostly to stress the point of how much you take such a system for granted when it's available and just works - since I've moved here owning a car would be almost an inconvenience, and even London feels like a horrible public transportation experience.