  One of the main characteristics of the penumbra of sunspots is the radially
outward-directed Evershed flow. Only recently have penumbral regions been
reported with similar characteristics to normal penumbral filaments, but with
an opposite direction of the flow. Such flows directed towards the umbra are
known as counter Evershed flows (CEFs). We aim to determine the frequency of
occurrence of CEFs in active regions (ARs) and to characterize their lifetime
and the prevailing conditions in the ARs. We analysed the continuum images,
Dopplergrams, and magnetograms recorded by SDO/HMI of 97 ARs that appeared from
2011 to 2017. We followed the ARs for $9.6\pm1.4$ days on average. We found 384
CEFs in total, with a median value of 6 CEFs per AR. CEFs are a rather common
feature, they occur in 83.5% of all ARs regardless of the magnetic complexity
of the AR. However, CEFs were observed on average only during 5.9% of the mean
total duration of all the observations analyzed here. The lifetime of CEFs
follows a log-normal distribution with a median value of 10.6$_{-6.0}^{+12.4}$
hr. In addition, we report two populations of CEFs depending on whether they
are associated with light bridges, or not. We explain that the rarity of
reports of CEFs in the literature is a result of highly incomplete coverage of
ARs with spectropolarimetric data. By using the continuous observations now
routinely available from space, we are able to overcome this limitation.
