Metal ions play a vital role in many biological processes. An important factor in these processes is the dynamics of exchange between ion bound-water molecules and the bulk. Although structural and dynamical properties of labile waters bound to metal ions, such as Na+ and Ca2+, can be elucidated using molecular dynamics simulations, direct evaluation of rates of exchange of waters rigidly bound to high charge density Mg2+, has been elusive. Here, we report a universal relationship, allowing us to determine the water exchange. time on metal ions as a function of valence and hydration radius. The proposed relationship, which covers times spanning 14 orders of magnitude, highlights the ultra sensitivity of water lifetime to the ion size, as exemplified by divalent ions, Ca2+ (similar to 100 ps) and Mg2+ (similar to 1.5 mu s). We show that even when structures, characterized by radial distributions are similar, a small difference in hydration radius leads to a qualitatively different (associative or dissociative) mechanism of water exchange. Our work provides a theoretical basis for determination of hydration radius, which is critical for accurately modeling the water dynamics around multivalent ions, and hence in describing all electrostatically driven events such as ribozyme folding and catalysis.