It is demonstrated that the non-linear bubble growth in Rayleigh-Taylor (RT) unstable laser targets is quasi-static and obeys a gt2 law. Smoothing mechanisms acting on the spikes, such as tip broadening or tip ablation, will not stabilise the RT growth once bubble growth is established, hence shell lifetime is determined by bubble growth in the target. The authors also show that mass perturbations of initial amplitude A and wavelength lambda satisfies A/lambda >0.04 will grow according to a gt2 law almost immediately, with no observable linear growth phase. Results are presented from 2D computer simulations of spherical laser targets which support the conclusions.