How long will it really take to build a decent income working online?
A decent income is something that has to be defined by what you intend to spend your income on.
The bulk majority of people who choose to work online, do so, not so much because they are driven by the prospects of vast amounts of money (although yes, that is a possibility) but because working online allows them the flexibility to dictate when and how much they want to work.
It allows them to freedom to spend time with their families and the ability to work from anywhere in the world that there is an internet connection (which is pretty much anywhere in the world).