  We consider a scale-invariant helical magnetic field generated during
inflation. We show that, if the mean magnetic helicity density of such a field
is measured, it can be used to determine a lower bound on the duration of
inflation. Upper bounds can be used to derive constraints on the minimal
duration of inflation if one assumes that the magnetic field generated during
inflation is helical. Using three-dimensional simulations, we show that an
initially scale-invariant field develops, which is similar both with and
without magnetic helicity. In the fully helical case, however, the magnetic
field appears to have a more pronounced folded structure.
