No matter how many times I click "Update Now", I always see this message on the Dashboard > Updates page:
An updated version of WordPress is available.
This is simply a flaw of how we handle updates to nightly builds. Rather than issuing an update every 24 hours based on the new nightly build that was generated, it *always* says that there is an update available.
For automatic updates, that means sites will update every 12 hours (when the cron event fires for auto updates) when you are running a development version.
I think we're going to try to fix this in 3.8.
That said: You should never receive auto update notifications less than about 12 hours apart. (11 hours is possible as well.) But, if your site is just a test site that doesn't get any traffic, then it is possible that your cron didn't fire (which requires visits to the site) until you finally visited the site yourself. In that case, it is possible updates are delayed.
Remkus, if your site updated multiple times in the same hour, let's say, then that's a problem. But note that this should have been rectified yesterday in http://core.trac.wordpress.org/changeset/25828.