  We construct realistic sensitivity curves for pulsar timing array searches
for gravitational waves, incorporating both red and white noise contributions
to individual pulsar noise spectra, as well as the effect of fitting to a
pulsar timing model. We demonstrate the method on both simulated pulsars and a
realistic array consisting of a subset of NANOGrav pulsars used in recent
analyses. A comparison between the results presented here and exact sensitivity
curves shows agreement to tens of percent. The resulting sensitivity curves can
be used to assess the detectability of predicted gravitational-wave signals in
the nanohertz frequency band in a fraction of the time that it would take to
compute the exact sensitivity curves.
