I'll start by saying that this is really a suggested change pretty much solely for the sake of realism, but anyway:
IMO, the fact that water in a cauldron "goes" anywhere just by being lit is kind of ridiculous. You might say that it's evaporating during use but given that we're talking about liters upon liters of water here, that explanation doesn't really satisfy me.
I suggest a more intuitive mechanism: rather than the quantity of water depleting with use, I suggest that the quality of the water decrease. Then, once it gets past a point no longer up to the standards of the user, the water can be changed.
Think of it like washing dishes in a sink: you start with clean water, but as you wash more and more dishes, the water gets dirtier, so you eventually have to drain it away and bring in new water (if you're washing a lot of dishes, that is).
Again, I can't really defend this idea on the basis of anything other than the fact that I think it would be a more realistic mechanism, but I still thought I'd share.