I think it's more useful to estimate the amount of rock (or "layers of rock") you will strip by using
volume of muriatic acid, rather than
length of time in bath. The acid will strip the outer layers of rock until neutralized [by the rock]. 30 minutes should be enough to finish such reaction; 24 hours is certainly enough. Theoretically, you should be able to stick your hand into the bath once the foaming/bubbling stops, since the reaction is complete and the pH has stabilized at a higher level, but you can be sure I wear a glove anyway
This post is meant to shed light on the reaction and how much acid to use. Stay safe