  Magnetic helicity has the remarkable property of being a conserved quantity
of ideal magnetohydrodynamics (MHD). Therefore, it could be used as an
effective tracer of the magnetic field evolution of magnetized plasmas.
Theoretical estimations indicate that magnetic helicity is also essentially
conserved with non-ideal MHD processes, e.g. magnetic reconnection. This
conjecture has however been barely tested, either experimentally or
numerically. Thanks to recent advances in magnetic helicity estimation methods,
it is now possible to test numerically its dissipation level in general
three-dimensional datasets. We first revisit the general formulation of the
temporal variation of relative magnetic helicity on a fully bounded volume when
no hypothesis on the gauge are made. We introduce a method to precisely
estimate its dissipation independently of the type of non-ideal MHD processes
occurring. In a solar-like eruptive event simulation, using different gauges,
we compare its estimation in a finite volume with its time-integrated flux
through the boundaries, hence testing the conservation and dissipation of
helicity. We provide an upper bound of the real dissipation of magnetic
helicity: It is quasi-null during the quasi-ideal MHD phase. Even when magnetic
reconnection is acting the relative dissipation of magnetic helicity is also
very small (<2.2%), in particular compared to the relative dissipation of
magnetic energy (>30 times larger). We finally illustrate how the helicity-flux
terms involving velocity components are gauge dependent, hence limiting their
physical meaning.
