Depends on how you have ranked it. Sometimes they are sandboxed n1 just for a couple of hours to check if the CTR is according to the query. It's common that they drop afterwards.
Also, if you have blasted them, it's extremely common that they stay a little bit on the top, and then they drop like mad.
If you have developed a good SEO strategy either with BH or WH mechanisms, it shall not only stay, but maintained if you keep on going and monitoring your competence.
Personally lately I've been testing a dual-rank with a second money site for certain interesting keywords. So 1st rank is my main money site and 2nd rank is a secondary money site without a footprint except that it actually links to the first one, making it even more solid.