placing 1 fuel into it gives you 15mins of time, but it seems that if you place more than 1 in at one time it will only go asfar as 15mins worth.
lamo terms: place 1 in and get 15 mins. but place more than 1 in at the same time and you get only 15 mins still. however if you do them all individually its fine and you get more mins stacked.