  The time evolution of the buildup process inside a double-barrier system for
off-resonance incidence energies is studied by considering the analytic
solution of the time dependent Schr\"{o}dinger equation with cutoff plane wave
initial conditions. We show that the buildup process exhibits invariances under
arbitrary changes on the system parameters, which can be successfully described
by a simple and easy-to-use one-level formula. We find that the buildup of the
off-resonant probability density is characterized by an oscillatory pattern
modulated by the resonant case which governs the duration of the transient
regime. This is evidence that off-resonant and resonant tunneling are two
correlated processes, whose transient regime is characterized by the same
transient time constant of two lifetimes.
