Change the water every time it reaches a light yellow/green (and I do mean light) this may be 3-4 times first day or two. You are using osmosis to move around things in the plant (not thc since not water soluble) but the chlorophylls and various other water soluble things out into the water. Higher concentrations move to areas of lower concentrations. There is a high concentration inside the cells of the plants and one of 2 things happen here (somebody needs to pull out a REAL microscope plz? lol). Either the water soluble compounds move across the cell and into the water, or the cells are absorbing water en mass to equalize the concentrations on either side of the cell wall and bursting, releasing out these same compounds.
Anyways - back to this, just change water as often as possible until water starts staying clean, and dry as quickly as possible to avoid mold issues, maybe put it inside a plastic/glass jar with a thin cloth wrapped over the mouth and sides so heat isnt great, but does its job.
Note - I assume you know this already, but water curing gets rid of alot of flavors and smells associated with weed. And it will weigh less than a bud of comparable size that was cured the old fashioned way due to loss of those compounds.
