Cross-correlation significance in R

How do you tell if the correlations at different lags obtained from the cross-correlation (ccf function) of two time series are significant.

The variance of the cross-correlation coefficient under the null hypothesis of zero correlation is approx $1/n$ where $n$ is the length of the series. The coefficients are also asymptotically normal. So approximate critical values (at the 5% level) are $\pm 2/\sqrt{n}$.
These critical values are plotted automatically in R using ccf(x,y).