Consider the following sentence:
I ate today.
It can easily translate as:
Now consider the following:
How much did you eat today?
In the last sentence, there's a subtle hint of "how much did you eat for today?" Somewhat like, say, you are on a diet and must count your calories. So the doctor asks you about your intake: "How much today?", "How much was it yesterday?". Can this context be translated with a por?
¿Cuanto comíste por hoy?
Please let me know if you need me to explain this better. Not sure if I am being able to convey the context clearly here.