  The detection of new clusters of galaxies or the study of known clusters of
galaxies in X-rays can be complicated by the presence of X-ray point sources,
the majority of which will be active galactic nuclei (AGN). This can be
addressed by combining observations from a high angular resolution observatory
(such as Chandra) with deeper data from a more sensitive observatory that may
not be able to resolve the AGN (like XMM). However, this approach is undermined
if the AGN varies in flux between the epochs of the observations. To address
this we measure the characteristic X-ray variability of serendipitously
detected AGN in 70 pairs of Chandra observations, separated by intervals of
between one month and thirteen years. After quality cuts, the full sample
consists of 1511 sources, although the main analysis uses a subset of 416
sources selected on the geometric mean of their flux in the pairs of
observations, which eliminates selection biases. We find a fractional
variability that increases with increasing interval between observations, from
about 0.25 for observations separated by tens of days up to about 0.45 for
observations separated by $\sim 10$ years. As a rule of thumb, given the
precise X-ray flux of a typical AGN at one epoch, its flux at a second epoch
some years earlier or later can be predicted with a precision of about $60\%$
due to its variability (ignoring any statistical noise). This is larger than
the characteristic variability of the population by a factor of $\sqrt{2}$ due
to the uncertainty on the mean flux of the AGN due to a single prior
measurement. The precision can thus be improved with multiple prior flux
measurements (reducing the $\sqrt{2}$ factor), or by reducing the interval
between observations to reduce the characteristic variability.
