We always carry a second full 907 bottle with us, although this is in general not necessary at all.
Follow this math:
Empty bottle weighs: 3850 Gram
Maximum filling weight (gas): 2750 Gram
So, a full bottle weighs: 6,6 Kilo.
One burner uses max 114 Gram of gas per hour.
Two burners use 228 Gram per hour.
Making tea, baking eggs, cooking macaroni on one burner gives you 2750 / 114 = 20,7 burning hours.
If you are a scrooge like me, and turn the gas button to a minimum (no, not anti-clockwise, but clockwise to the far right; simmering), then you can easily double the burning hours to 40 (for one burner). And then: my tea and macaroni are boiling in minutes, not in hours. So even a half full bottle may last for a trip of 4 weeks.
Using a spring balance: anything above 3,85 Kilo (= empty bottle) means you can still make (some) tea that day.
More practical is: you only need to shake the bottle to know the truth: do you hear gas splashing, then you're save for that day.
Btw: I found out that my 907 Campingaz bottles have different tare weights (= the empty bottle itself) stamped in the iron ring at the bottom. My new one says: 3,85 Kilogram, an older one has 4 KG stamped in it. The stated contents may also vary, from 2750 tot 2950 or even 3000 gram of gas. So, best to stay on the safe side.
A tip for next years holiday: change your empty gas bottle on the continent, in Holland I find a filling for 25 euros, in Italy this year I paid even less: 21 euro.
Filled regards from Amsterdam,
Marc.