If comparing to English, когда ты хочешь would be always, usually, often, so it's something of a regular situation, something of Present Simple, more of a general talk about something (it's not right now, it happens in general). Когда ты хочешь завтракать, хорошо бы, чтобы была еда в холодильнике. - When you want to have breakfast, that is nice when there is some food in the fridge. Когда ты захочешь, would be 'when you finally want it', 'when you are ready to have it', I guess it is used more in a certain special situation. - Okay, when you're ready for your breakfast, it's gonna be down there in the kitchen.