I would say that you should look for the output. The original charger has an output of 1200ma.
The minimum output of a usb connector is by default 500ma. At 500ma the charging would be much longer.
Some phones do not even charge at 500ma.
I would suggest you look for at least 1000ma.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?