Fill it some normal-looking amount. It won't evaporate that quickly; it's a pretty large volume compared to the surface area. If you want it to evaporate more quickly, you need to increase the surface area exposed to the air, which can be done by adding some material that wicks such as fabric, paper towel, moss, etc. Then all the convolutions in the material's surface essentially become additional surface area from which water can evaporate.
In any case, it will probably take some trial and error because all of this depends on the temperature and humidity in your house and how conditions change. What works now might not work six months from now when the weather is different, and conditions in your house are different. But don't worry; even if the water bowl dries up while you're at work, your snake will be without water for a few hours at most and then you'll come home and refill it.![]()