AIUI, H2O disassociates fairly seldom, but when it does, it does so fast. And re-associates even faster, but not necessarily with the same individual atoms. As an indication of how fast that happens, it happens on average to every molecule of water every ten hours, but (from the definition of pH)
a quantity of 107 moles of pure (pH 7) water, or 180 metric tonnes (18×107 g), contains close to 18 g of dissociated hydrogen ions.
at every given moment. But this at an extremely fast turnover rate, those 18 g of H+ (and the corresponding 306 g of OH-) change all the time. It must be fast, or it couldn’t happen to every molecule every ten hours on average.
Now imagine that this is happening inside your body too, all the time.