Everything depends on a product you sale. I do digital products myself.
Once I spent several months developing one of my products and received the first sale after a week from the launch day. It was actually the only sale I made that time..
I now prepare a short sales page on a product home page with opt-in. Try doing some social marketing and pumping expectation together with development. As soon as I launch I have several early-birds ready to buy or test and then spread the word.
Anticipation and buzz – my key principles.