Work on the most important features and release them when they are ready.
There isn't a magic time frame to work to.
Pre-determined upgrade cycles make sense for complex software like an Mac OS and Ubuntu. But once your web startup has matured, there might not be much of a need for constant iterating. Think of your favorite web apps, how often do they iterate? Gmail? Instapaper? Delicious? And if you're running a "directory-like" web site, I really don't see how you can continue to add so many new features you need to set a release schedule in advance.
Just make tweaks as needed. Encourage your users to offer feedback. Perhaps you could use UserVoice to have them submit and vote on suggestions. Also, make sure you remain cognizant of how startup's value proposition and make sure that your users are extracting value from your product. (Send personalized emails to new/old/random users to find out how they are using your web site.)
P.S. Companies often forget that "User Experience" is a feature. Make your site easier and more pleasant to use can be achieved by simply increasing the size of a headline or a button. Make sure you focus on those kinds of things too.
I'd propose a different viewpoint : How much can you remove and retain the services usefulness?
Each time you say yes to a feature, you’re adopting a childFrom 37 Signals: Getting Real - Start with No Biggest takeaway: "Innovation is not about saying yes to everything. It's about saying NO to all but the most crucial features."