The idea that self-control (or executive) functions depend on limited "mental resources"that can be depleted (aka ego-depletion) has generated a lot of interest, but both the empirical status of the phenomenon and its theoretical explanation remain controversial. Here, we tested a widely neglected but straightforward prediction of ego-depletion theory: The longer people work on a control-demanding task, the more should their ego deplete. If so, ego-depletion effects should become more pronounced as time on (control) task increases. To test that prediction, we carried out an online experiment, in which participants switched between blocks of a numerical Stroop task (NST) with either 50% or 10% incongruent trials, which served to induce different degrees of ego depletion, and a Global-Local Task (GLT), which served to measure the impact of ego depletion. We predicted that participants would perform more poorly on the GLT if it is combined with the more demanding NST and that this performance cost would systematically increase over time on task. Although the classical Stroop and global-local effects were replicated, we found no evidence that our experimental manipulation successfully induced an outcome that can be considered as evidence for ego depletion. We conclude that our findings contribute to the growing literature questioning the robustness of ego-depletion effects under certain task conditions.