  We use numerical simulations to test a broad range of plausible observational
strategies designed to measure the time delay between the images of
gravitationally lensed quasars. Artificial quasar light curves are created
along with Monte-Carlo simulations in order to determine the best temporal
sampling to adopt when monitoring the photometric variations of systems with
time delays between 5 and 120 days, i.e., always shorter than the visibility
window across the year. Few and realistic assumptions are necessary on the
quasar photometric variations (peak-to-peak amplitude and time-scale of the
variations) and on the accuracy of the individual photometric points. The
output of the simulations is the (statistical) relative error made on the time
delay measurement, as a function of 1- the object visibility over the year, 2-
the temporal sampling of the light curves and 3- the time delay. Also
investigated is the effect of long term microlensing variations which must be
below the 5 % level (either intrinsically or by subtraction) if the goal is to
measure time delays with an accuracy of 1-2 %. However, while microlensing
increases the random error on the time delay, it does not significantly
increase the systematic error, which is always a factor 5 to 10 smaller than
the random error. Finally, it is shown that, when the time delay is comparable
to the visibility window of the object, a logarithmic sampling can
significantly improve the time delay determination. All results are presented
in the form of compact plots to be used to optimize the observational strategy
of future monitoring programs.
