The most honest way to answer this is that he needs a certain amount of calories a day (or 7 times that in a week). Formulas drive a lot of people crazy, but it is Weight (in kilograms)^0.75 x 72= daily calorie need for an active tortoise. Drop it in half for an inactive tortoise (warm temps, older and not breeding, etc.) and boost it by about half for a very active tortoise (preparing for brumation, mating, laying eggs, etc.) About half the calories should come from carbs, the other half split evenly over fats and proteins. (For grassland tortoises, the ratio is 75% carbs, 20% proteins, 5% fats.)
A good diet plan offers your tortoise plenty of high-fiber and -calcium, but low-carbohydrates, fats, and proteins foods, then a much smaller amount of richer foods. Some keepers do this by portion control, some by using fasting days, some by removing foods, etc.
A wild tortoise uses two basic strategies- wander and graze on low-value plants a lot during the day, or gorge on high-value plants and sleep it off. This varies by season, plant availability, sheer luck, and so on.
In captivity, they get richer foods, less exercise, little seasonal variation... so you need to find a way to offer the foods they need in a way that works for you.
I have 6 red foots that are between 10 and 13inches I normally feed them every day a week. but I have read in some places that you should not feed adults every day, and that they should be feed every other day.
I feed Rosie 6 days a week. Sometimes daily. After she gets Mazuri or plain chicken/worms for protein she usually won't eat as much the next day so I wait a day and she devours it the next. But she's 2 years old and typically has a voracious appetite. She does also munch on the plants in her enclosure from time to time too.