  ALMA observations of the Serpens South star-forming region suggest that
stellar protoclusters may be completely mass segregated at birth. Independent
observations also suggest that embedded clusters form segregated by mass. As
the primordial mass segregation seems to be lost over time, we aim to study on
which timescale an initially perfectly mass-segregated star cluster becomes
indistinguishable from an initially not mass-segregated cluster. As an example,
the Orion Nebula Cluster (ONC) is also discussed. We used $N$-body simulations
of star clusters with various masses and two different degrees of primordial
mass segregation. We analysed their energy redistribution through two-body
relaxation to quantify the time when the models agree in terms of mass
segregation, which sets in only dynamically in the models that are primordially
not mass segregated. A comprehensive cross-matched catalogue combining optical,
infrared, and X-ray surveys of ONC members was also compiled and made
available. The models evolve to a similar radial distribution of high-mass
stars after the core collapse (about half a median two-body relaxation time,
$t_\mathrm{rh}$) and become observationally indistinguishable from the point of
view of mass segregation at time $\tau_\mathrm{v} \approx 3.3\,t_\mathrm{rh}$.
In the case of the ONC, using the distribution of high-mass stars, we may not
rule out either evolutionary scenario (regardless of whether they are initially
mass segregated). When we account for extinction and elongation of the ONC, as
reported elsewhere, an initially perfectly mass-segregated state seems to be
more consistent with the observed cluster.
