The slower you charge it, the less it damages the battery. Ideally your charge current would be <0.5C, which means it should take at least 2 hours to go from 0% to ~90% charge. A typical 500mA USB 2.0 port on a computer is often a convenient way to accomplish this.
Unless you NEED your phone to charge up as fast as possible because you're in a hurry, those crazy high power high speed charging solutions should be avoided.