  Central engine of gamma-ray bursts (GRBs) may be intermittent and launch
several episodes of ejecta separated by a long quiescent interval. In this
scenario, an external shock is formed due to the propagation of the first
launched ejecta into the circum-burst medium and the later launched ejecta may
interact with the external shock at later period. Owing to the internal
dissipation, the later launched ejecta may be observed at a later time
($t_{\rm{jet}}$). In this paper, we study the relation of $t_{\rm{b}}$ and
$t_{\rm{jet}}$, where $t_{\rm{b}}$ is the collision time of the later launched
ejecta with the formed external shock. It is found that the relation of
$t_{\rm{b}}$ and $t_{\rm{jet}}$ depends on the bulk Lorentz factor
($\Gamma_{\rm{jet}}$) of the later launched ejecta and the density ($\rho$) of
the circum-burst medium. If the value of $\Gamma_{\rm{jet}}$ or $\rho$ is low,
the $t_{\rm{b}}$ would be significantly larger than $t_{\rm{jet}}$. However,
the $t_{\rm{b}}\sim t_{\rm{jet}}$ can be found if the value of
$\Gamma_{\rm{jet}}$ or $\rho$ is significantly large. Our results can explain
the large lag of the optical emission relative to the $\gamma$-ray/X-ray
emission in GRBs, e.g., GRB~111209A. For GRBs with a precursor, our results
suggest that the energy injection into the external shock and thus more than
one external-reverse shock may appear in the main prompt emission phase.
According to our model, we estimate the Lorentz factor of the second launched
ejecta in GRB~160625B.
