Regolith generation by thermal fatigue has been identified as a dominant mechanism for the breakdown of small (cm-sized) rocks on certain airless bodies. Simple numerical models for thermal fatigue seemed to indicate that this breakdown occurs faster in the larger decimeter-sized rocks, which is in contrast to the predictions of disruption models through successive mi- crometeorite impacts. The observation is justified by the existence of larger temperature gradient in bigger rocks, but it is not clear that this conclusion can be extrapolated or scaled to meter-sized boulders. Here we reveal a transition in the rock disaggregation rates by thermal fatigue when rock sizes rise above a critical length scale. A simple analytic model is formulated to predict the time to fracture of rocks on small airless bodies. We con- sider an uncoupled approach consisting of a one-dimensional thermal model, and a two-dimensional fracture model. The solution of the heat equation is used as input to the thermomechanical crack growth problem. This new understanding could provide bounds on the survival rates of asteroidal rocks, and may help in coupling thermal fatigue with a mechanical disruption model to obtain a multi-mechanism view of regolith evolution in the solar system.