I am a website owner and I have copyrighted (copywritten?) my website.
If I continue to add pages to it, how often (if ever) do I have to submit the new pages to the copyright office (along with the form and the $30 fee)?
In the USA it’s generally regarded as acceptable to merely post ©year author and not have to really register anything.
Anything else seems like a waste of money.
On the other hand, what are you protecting? And what’s your URL? And which pages aren’t registered yet? 
All new pages are automatically copyrighted from the moment you write them. Not even from the moment you post them, the moment you write them. You don’t even have to post the copyright symbol any more.
Registration with the Copyright Office is meaningful only if you become involved in a lawsuit. It allows you more awards if you win a case.
I am a staunch supporter of Copyright. But the odds of your ever being involved in a lawsuit in which registration is vital are so small that I think you are throwing your money away. However, if you still want to do this, note you can register your pages up to 90 days after posting and still have them covered in case of lawsuit. Therefore you can register all the pages you do every quarter for one fee of $30.
Go to the site I linked to at the Copyright Office for more information.
Clarification: The past tense of “copyright” is “copyrighted.” Copyright is the right to copy. “Written” is the past tense of the verb “write,” which is not part of the verb “copyright.”
Balthisar gets it wrong – as Exapno states, there’s no need to take affirmative action of any kind to protect your pages. There are two major benefits to registering in advance. 1) The Copyright office has a record of your work at the date you registered, making it easy to prove that you didn’t steal something from him after he published it. 2) If your work is unregistered when it’s stolen, you lose out on the really good money damages (but you still have the right to stop the thief from using your stuff.)
–Cliffy
So then cliffy, it would be worth the time and money to copyright new pages periodically?
I am neither competent nor willing to answer specific legal questions about intellectual property law.
–Cliffy
IANAL, so I can make broad generalizations that the people who are really qualified to answer aren’t willing to commit to (with the failsafe that Cliffy’s ethical obligation not to give legal advise doesn’t extend to calling BS on me if I’m wrong).
Note that the difference between registered and unregistered copyrights is the ability to recover damages. Now consider the kind of material you’re publishing on your website and whether damages are realistic. If you’re publishing works of your own creation such as a novel you’ve written or a newsletter you create, you stand to suffer considerable financial damages if someone copies your work, especially if your website is generating revenue from your content through subscriptions, advertising, etc. If your website is less creative and more like a compilation of links or assembled content from other sources (e.g. something like Joe Bob’s Big Truck Fact Page), or if your site is comprised of content that you could never derive substantial revenue from (e.g. bad family photos that no one but your mother would ever care about), then it’s less likely you could demonstrate real damages if someone stole your work. As Cliffy pointed out, whether you’re registered or not you can keep someone from reusing your copyrighted works, so whether you want to register should be based on whether you could demonstrate to a court that you suffered real financial damages from unauthorized use.