Camping for 2 Ive found the cartridges last about 2 - 3 days of course depending on how much you use them, normally have open fires if i can and only take one for convenience or as a back up if the weather is bad
My tip of the day however is to get the cartridges from Tescos at the end of the summer, they are usually sold off at a pound or so each as they are not prepared to store any stock over the winter months - you finds the same with fireworks, normally buy mine a few days after bonfire night. You gotta be quick though as all the coarse fishermen already seem to know the trick so you'll have a window of a couple of days at best as they are a bargain!!