For most affiliate marketers who are new to this game, promoting an affiliate program mostly means setting up a few affiliate links on their site and then wait for the cash to start rolling in. Unsurprisingly in 90% of the cases, this strategy fails because their site doesn't get enough traffic or because they don't take the time to make this work, letting their affiliate links sleep instead of testing various factors that might increase their sales - there are also a few cases where affiliate programs don't work but if you go with popular programs you shouldn't encounter that problem.
Of course if you work full-time on websites, you already know how much time is needed to become successful but, for all of you who are promoting affiliate offers on your spare time to get an additional source of income, I just wanted to clarify this: Having met a few successful affiliates and running a quite profitable website on my free time myself, I can tell you from experience that it requires at least a couple of hours a day of work to become successful at this game.
While this work might not every day be directly related to promoting affiliate programs, working on a daily basis for a couple of hours to improve your website's content, traffic, navigation or affiliate revenues will inevitably result in your sales increasing. With that type of effort, success might not occur overnight but it will come and this is what will makes the difference with the zillion of other affiliate sites that fail miserably.