Wayne.....it is a bit like how long is a piece of string? It all depends on how much cooking or heating with gas you do.
We spent 72 nights in Spain this year and took 2x6kg full Calor Propane cylinders with us. We actually used about 1 and a half of these.
However, we made full use of the 16amp electricity and we did a lot of cooking on a George Foreman Grill, an Electric Frying pan and a slow cooker. Our Caravan oven also has 3 gas and one electric ring so we made the most use of electricity as possible.
We rarely used the heating and then only on electric, also our Fridge and water heating was electric.
I would be careful about getting your Calor cylinder refilled though, there have been reports that it is not always safe. If you run out I would, as suggested, switch to Camping Gas but you will need to ensure you have the correct regulator.