http://www.theatlantic.com/business/archive/2011/04/how-to-start-your-own-private-currency/73327/

"Architects of private currencies face a chicken-egg challenge. To make a new currency popular among users, you have to sign up retailers. To make it popular among retailers, you have to sign up users."

"The most interesting currency facing this existential crisis is BitCoin, a computer-generated currency that is famously traded among a couple hundred computer geeks and merchants. 'The only thing valuable about BitCoin is that there are other nerdy internet denizens that will accept it for payment for things,' Blanchard said."

"BitCoin is backed by neither metals nor man-hours, yet each coin is worth nearly a dollar due to demand for the scarce BitCoins that have been released among the network of users. BitCoin is currently legal and active. But some worry it could face a fate like E-Gold, the bullion-backed online currency that quickly became a favorite of global crooks and money launderers. Its founder was sentenced to house arrest."

This was four years ago; I'm wondering what 'long enough' is for any currency to be legitimized for the masses.