We investigate the time evolution of an impulsive start problem for arbitrary Knudsen numbers ($\hbox{\it Kn}$) using a linearized kinetic formulation. The early-time behaviour is described by a solution of the collisionless Boltzmann equation. The same solution can be used to describe the late-time behaviour for $\hbox{\it Kn}\,{\gg}\,1$. The late-time behaviour for $\hbox{\it Kn}\,{<}\,0.5$ is captured by a newly proposed second-order slip model with no adjustable parameters. All theoretical results are verified by direct Monte Carlo solutions of the nonlinear Boltzmann equation. A measure of the timescale to steady state, normalized by the momentum diffusion timescale, shows that the timescale to steady state is significantly extended by ballistic transport, even at low Knudsen numbers where the latter is only important close to the system walls. This effect is captured for $\hbox{\it Kn}\,{<}\,0.5$ by the slip model which predicts the equivalent effective domain size increase (slip length).