For toads and many other greedy frogs I use the 15 minute rule - as much as they will eat in 15 minutes or so. For young toads or frogs, particularly very young ones, I feed every day. For older ones and adults I would feed twice a week, or one larger feed per week for fully grown individuals. You can always vary it. Young specimens shouldn't be able to get obese - it's the older ones you have to worry about.