You just grab the cooking
device you have and read its specs. Manufacturers clearly quote how much gas the unit consumes per hour.
You will allow for both your lifestyle (some eat everything cooked, some eat everything raw, look up Webb Chiles' blog for more) as well as for varying wx requirements. No tea and coffee in the tropics perhaps, but plenty in the Southern, etc.
In our boat
of two, in the tropics and sub-tropics, we seem to use about 3 kg of (butane) gas each month. This would translate into 1.5 kg of gas per person per month in hot climates. This is when we are full time out (on a crossing or anchored, NOT at the dock).
If you count on 300 days at sea, you may want to carry at least 25 kg of cooking gas. Or 4 x 6 liter bottles. Plastic bottles will save plenty of weight.
Double check my maths prior to any commitments.
Get a twin hob stove. Hobs tend to die on you at times.
Get a spare regulator
. Regulators tend to die on you at times.