  Formation time of heavy quarkonia in a homogeneous magnetic field is analyzed
by using a phenomenological ansatz of the vector current correlator. Because
the existence of a magnetic field mixes vector quarkonia ($J/\psi$,
$\psi^\prime$) and their pseudoscalar partners ($\eta_c$, $\eta_c^\prime$), the
properties of the quarkonia can be modified through such a spin mixing. This
means that the formation time of quarkonia is also changed by the magnetic
field. We show the formation time of vector quarkonia is delayed by an
idealized constant magnetic field, where the formation time of the excited
state becomes longer than that of the ground state. As a more realistic
situation in heavy-ion collisions, effects by a time-dependent magnetic field
are also discussed, where delayed formation of $J/\psi$ and $\psi^\prime$ and
very early formation of $\eta_c$ and $\eta_c^\prime$ are found.
