Time-Varying Bayesian Optimization (TVBO) is the go-to framework for optimizing a time-varying black-box objective function that may be noisy and expensive to evaluate. Is it possible for the instantaneous regret of a TVBO algorithm to vanish asymptotically, and if so, when? We answer this question of great theoretical importance by providing algorithm-independent lower regret bounds and upper regret bounds for TVBO algorithms, from which we derive sufficient conditions for a TVBO algorithm to have the no-regret property. Our analysis covers all major classes of stationary kernel functions.