My rule of thumb is around 12 grams gas for each litre of water you need to boil. So if you boiled three litres per day (say 2 cuppas and one rehydrated meal per person) for eight days, around 300 grams gas. Many variables including air and water temps, windiness and wind-shield efficiency (cooking outside in any breeze wastes a lot of gas), stove type, and pot type. Simmering requires much less gas per minute. Probably worth having a 30% margin of error for mishaps, emergencies, wastage, helping other hikers, medical use of hot water, and so on, carried in two separate canisters. Liquid fuel (eg metho) is about double, ie 25 grams per litre.
Hope this helps,