  Accretion in young stellar objects (YSOs) is at least partially episodic,
i.e. periods with high accretion rates ('bursts') are interspersed by quiescent
phases. These bursts manifest themselves as eruptive variability. Here we
present a systematic survey for eruptive YSOs aiming to constrain the frequency
of accretion bursts. We compare mid-infrared photometry from Spitzer and WISE
separated by ~5 yr for two samples of YSOs, in nearby star forming regions and
in the Galactic plane, each comprising about 4000 young sources. All objects
for which the brightness at 3.6 and 4.5 mu is increased by at least 1 mag
between the two epochs may be eruptive variables and burst candidates. For
these objects, we carry out follow-up observations in the near-infrared. We
discover two new eruptive variables in the Galactic plane which could be FU
Ori-type objects, with K-band amplitudes of more than 1.5 mag. One object known
to undergo an accretion burst, V2492 Cyg, is recovered by our search as well.
In addition, the young star ISO-Oph-50, previously suspected to be an eruptive
object, is found to be better explained by a disk with varying circumstellar
obscuration. In total, the number of burst events in a sample of 4000 YSOs is
1-4. Assuming that all YSOs undergo episodic accretion, this constraint can be
used to show that phases of strong accretion (>10^-6 Msol/yr) occur in
intervals of about 10^4 yr, most likely between 5000 and 50000 yr. This is
consistent with the dynamical timescales for outflows, but not with the
separations of emission knots in outflows, indicating that episodic accretion
could either trigger or stop collimated large-scale outflows.
