  We analyze numerical-relativity (NR) waveforms that cover nine orbits (18
gravitational-wave cycles) before merger of an equal-mass system with low
eccentricity, with numerical uncertainties of 0.25 radians in the phase and
less than 2% in the amplitude; such accuracy allows a direct comparison with
post-Newtonian (PN) waveforms. We focus on one of the PN approximants that has
been proposed for use in gravitational-wave data analysis, the restricted 3.5PN
``TaylorT1'' waveforms, and compare these with a section of the numerical
waveform from the second to the eighth orbit, which is about one and a half
orbits before merger. This corresponds to a gravitational-wave frequency range
of $M\omega = 0.0455$ to 0.1. Depending on the method of matching PN and NR
waveforms, the accumulated phase disagreement over this frequency range can be
within numerical uncertainty. Similar results are found in comparisons with an
alternative PN approximant, 3PN ``TaylorT3''. The amplitude disagreement, on
the other hand, is around 6%, but roughly constant for all 13 cycles that are
compared, suggesting that only 4.5 orbits need be simulated to match PN and NR
waves with the same accuracy as is possible with nine orbits. If, however, we
model the amplitude up to 2.5PN order, the amplitude disagreement is roughly
within numerical uncertainty up to about 11 cycles before merger.
