"Persistence pays off." It's a very old and often used aphorism, but it's definitely true, particularly when applied to the internet.
Take blogging, for instance. If you only write a 200 word post every couple of days consistently for a year, then you'll have over a hundred posts up. That's 20 000 words! Even if you did no link building whatsoever, you would still be drawing in some regular traffic just from that.
But imagine if you combined that blogging rate with a persistent link building campaign, such as one article a month and 5 free directory submissions. After a year you'd have 72 relevant backlinks.
Of course, the level of traffic depends on many other factors as well. But that amount of content with that number of backlinks would certainly bring in a respectable number of hits through search engines. Yet the work required wasn't much. (It would be if you had to do it all in a month or so!) The key is to do a little bit every day, or most days. And keep at it.
But if you work only very occasionally, you'll see little or no benefit. You could easily become disheartened, and give up. The blogosphere is littered with such blog corpses. Sad, because they could still be alive, kicking and very popular if their owners had just kept at it a bit more consistently.