As far as I know, accuracy of NTP syncronization highly depends on the network. I've seen some numbers from 50 microseconds to a "below one second" over the internet. Well, this is a huge difference.
I believe, accuracy dependence is a great question to study, but so far I failed to find any material, which clearly states that, say, some particular configuration grants that particular accuracy.
It is said on http://www.ntp.org/ntpfaq/NTP-s-algo.htm:
A time difference of less than 128ms between server and client is required to maintain NTP synchronization. The typical accuracy on the Internet ranges from about 5ms to 100ms, possibly varying with network delays. A recent survey[2] suggests that 90% of the NTP servers have network delays below 100ms, and about 99% are synchronized within one second to the synchronization peer.
With PPS synchronization an accuracy of 50µs and a stability below 0.1 PPM is achievable on a Pentium PC (running Linux for example).
That's something, but maybe there is some more thorough analysis on the topic?
Best Answer
No-one can guarantee how well NTP will work on your network, because no-one knows how well connected your network is to the internet, and to the clock servers thereon. However, according to the clock discipline algorithm page on ntp.org
Note that large-but-stable latency between your LAN and the internet's clock servers doesn't have as bad an effect on accuracy as highly-variable latency.
You don't say where you got the estimates above ('50 microseconds to ... "below one second"'), so I can't comment on them, but in my experience 50us is unlikely unless you have a directly-attached clock source, and 1s is unlikely unless you have a piece of wet string connecting you to the internet and you're using upstream servers in Antarctica.
Edit: the text you now quote in your question gives a pointer to a paper which, in 1999, did indeed establish that 99% of ntp servers are synch'ed to within one second. Fortunately, there is more recent work; in this paper some authors from the Federal University of Parana, Brazil, repeated the experiment in 2005, and found (if I understand their Fig. 1 correctly) that north of 99% - more like 99.5% - of servers now have offsets less than 100ms, and that 90% have offsets less than 10ms. This fits in pretty well with my experiences (see above).
Edit 2: one last wrinkle: all these studies don't investigate how accurate local clock is, but instead how far it differs from the upstream reference clock. These are patently not the same thing. But the first is unknowable; to know how wrong your clock is, you have to know exactly what time it is, and if you knew that, why would you have set your clock wrong in the first place? Just be aware that what these studies are measuring is not the difference between local clock and absolute time, but between local clock and reference clock.