  Variability is a characteristic feature of young stellar objects (YSOs) and
could contribute to the large scatter observed in HR diagrams for star forming
regions. For typical YSOs, however, the long-term effects of variability are
poorly constrained. Here I use archived near-infrared photometry from 2MASS,
UKIDSS, and DENIS to investigate the long-term variability of high-confidence
members of the four star forming regions Rho-Oph, ONC, IC348, and NGC1333. The
total sample comprises more than 600 objects, from which ~320 are considered to
have a disk. The dataset covers timescales up to 8 yr. About half of the YSOs
are variable on a 2sigma level, with median amplitudes of 5-20%. The fraction
of highly variable objects with amplitudes >0.5 mag in at least two
near-infrared bands is very low - 2% for the entire sample and 3% for objects
with disks. These sources with strong variability are mostly objects with disks
and are prime targets for follow-up studies. The variability amplitudes are
largest in NGC1333, presumably because it is the youngest sample of YSOs. The
frequency of highly variable objects also increases with the time window of the
observations (from weeks to years). These results have three implications: 1)
When deriving luminosities for YSOs from near-infrared magnitudes, the typical
error introduced by variability is in the range of 5-20% percent and depends on
disk fraction and possibly age. 2) Variability is a negligible contribution to
the scatter in HR diagrams of star forming regions (except for a small number
of extreme objects), if luminosities are derived from near-infrared magnitudes.
3) Accretion outbursts with an increase in mass accretion rate by several order
of magnitudes, as required in scenarios for episodic accretion, occur with a
duty cycle of >2000-2500 yr in the Class II phase. (abridged)
