Which is, incidentally, not at all a universal rule. I'd guess the majority of experienced brewers would prefer a 3-3 rule (3 weeks in primary, 3 in bottles--no secondary) if you have to do a timed rule, but it all depends on what beer you're brewing and what yeast you use.
Really, the key things are that:
1. You need to leave it in primary until it's done fermenting. A rule of thumb for that is to wait a week, then take a hydrometer reading every other day until you get 2 that are the same*.
2. After that, it helps to give it some time to condition. Opinions are split on whether you should move it to a secondary or leave it in the primary for this, though I think more people are leaning toward leaving it in primary and not using a secondary for basic beers (most people will still recommend a secondary if you're going to add fruit or oak chips, or if it's a really big beer that needs a long conditioning time).
3. After that, bottle it and let the bottles sit at 70F for 2-3 weeks. Open one up, if it's not done carbing then wait another week and check again.
*you should have some idea what the final gravity is supposed to be, and if you're way off then you might have a stuck fermentation rather than a finished primary
