This is one of the lesser known adages currently in use. I first heard it a short time ago when one of my friends was asked when a project he was working on would be finished. His response was "Real Soon Now." Two years later, the project was complete!
There is a hint of sarcasm inherent in the statement "Real Soon Now." If someone responded with "I will be done shortly" or "It shouldn't be much longer now," those expressions could be interpreted literally. With "Real Soon Now," though, it has become accepted that what is being referred to is nowhere near done. More than that, the speaker of the adage has no idea when it will be finished.
This expression should only be used with close friends. A report to your boss indicating that a crucial project will be done "Real Soon Now" will likely get you fired. Telling your significant other that you will commit to marriage "Real Soon Now" will likely lead to you being alone in life. A good example of when to use the phrase is if a friend asks you "When are you going to finish building that deck behind your house?" Your reply of "Real Soon Now" will likely lead to some laughter and, possibly, an offer of help!