  VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($
\theta_j \approx 4^\circ $) that dominated the afterglow peak at our viewing
angle, $ \theta_{\rm obs} \approx 20^\circ $. This implies that at the time of
the afterglow peak, the observed signal behaved like an afterglow of a top-hat
jet seen at $ \theta_{\rm obs} \gg \theta_j $, and it can be modeled by
analytic expressions that describe such jets. We use a set of numerical
simulations to calibrate these analytic relations and obtain generic equations
for the peak time and flux of such an afterglow as seen from various observing
angles. Using the calibrated equations and the estimated parameters of
GW170817, we estimate the detectability of afterglows from future double
neutron star mergers during the Advanced LIGO/Virgo observation run O3.
GW170817 took place at a relatively low-density environment. Afterglows of
similar events will be detectable only at small viewing angles, $ \theta_{\rm
obs} \lesssim 20^\circ $, and only $\sim 20\% $ of the GW detections of these
events will be accompanied by a detectable afterglow. At higher densities, more
typical to sGRB sites, up to $ 70\% $ of the GW detections are expected to be
followed by a detectable afterglow, typically at $ \theta_{\rm obs} \sim
30^\circ $. We also provide the latest time one should expect an afterglow
detection. We find that for typical parameters, if the jet emission had not
been detected within about a year after the merger, it is unlikely to be ever
detected.
