you don't tell ntpd how often to sync, it doesn't work like that, and it's kind of the case that you need to appreciate that it has reasons to do what it does.
ntpd will eventually slow down automatically to synchronize once every 1024 seconds, about 17 minutes. but if it is doing it more often than this then it has a good reason to. i found ntpd to be real black magic until i found the ntpq command. run "ntpq -pn" and you can see the current state of the ntp daemon. how often it's query the remote servers, which one's it's using etc... it's really an eye opener once you look at that.
that said i've had awful trouble getting a reliable time source under vmware and did have to use a cold hard ntpdate command to reign it in, but that's a very different story.
oh hey, there was a 2nd page!