Well, you can... but this isn't the best strategy.
Emergency contraception is meant only to be something to fall back on when conventional contraceptive methods (e.g. oral contraceptive pill, IUCD, Mirena) have failed. It isn't recommended that you use it as your primary method of contraception.
The reasons for this are twofold:
Firstly, the failure rate with emergency contraception is higher than most conventional contraceptive methods. Thus, if you are looking to prevent pregancy over the long run (as opposed to when other contraceptives fail), then Plan-B is actually worse than a lot of other products. There is a place for it or course, but that should only be when conventional contraceptives fail (e.g. broken condom).
Secondly, the dose of progesterone in Plan-B is actually higher than progesterone used in conventional contraceptives (e.g. combined oral contraceptive pill, Mirena). This means increased risk of side-effects (although they are usually fairly mild).
Hope that all helps :)