It depends on the number of roaches in the colony and what kind of container they are kept in. Ventilation plays a role in how often the habitat requires misting, in combination with temperature. In my large 50 gallon hisser aquarium, I lightly mist the roaches and also use a shallow water dish (currently a deli cup lid, upside down). I refill it every other day or so. I'm careful not to mist the substrate or dry pet food too much. I imagine many get their liquids from the fruits and veggies I put in. Roaches with a hard cuticle seem to be more resistant to drying out.
In small tupperware bins I use, I have very few holes for ventilation and only mist when the substrate seems bone dry. I imagine these roaches maintain their hydration through the lack of ventiliation in combination with the fruits and veggies.
Peter