  Aiming to study GRB engine duration, we present numerical simulations to
investigate collapsar jets. We consider typical explosion energy ($10^{52}$
erg) but different engine durations, in the widest domain to date from 0.1 to
100 s. We employ an AMR 2D hydrodynamical code. Our results show that engine
duration strongly influences jet nature. We show that the efficiency of
launching and collimating relativistic outflow increases with engine duration,
until the intermediate engine range where it is the highest, past this point to
long engine range, the trend is slightly reversed; we call this point where
acceleration and collimation are the highest "sweet spot" ($\sim$10 -- 30 s).
Moreover, jet energy flux shows that variability is also high in this duration
domain. We argue that not all engine durations can produce the collimated,
relativistic and variable long GRB-jets. Considering a typical progenitor and
engine energy, we conclude that the ideal engine duration to reproduce a long
GRB is $\sim$10 -- 30 s, where the launch of relativistic, collimated and
variable jets is favored. We note that this duration domain makes a good link
with Lazzati et al. (2013), which suggested that the bulk of BATSE's long GRBs
is powered by $\sim$10 -- 20 s collapsar engines.
