Not long at all. IME, it dissolves pretty fast. I have sometimes left a powerhead stirring it overnight, and sometimes the only mixing is pouring the water onto the CaO. The dissolution rate may depend on the source solids that you are using, but I would not agonize over it.
There are a few ways to tell if it's at saturation, but unless you know that you need saturation (i.e., less Ca++ and alkalinity leaves the tank with dropping levels of Ca++ and alk), that isn't an especially important fact to know.
Conductivity is the easiest way (IMO) to know if it's saturated. pH is another if you have a meter. Ca++ and alk measurement are also ways.