I've made a handful of these and find they put off more heat than store bought Sterno.
How long they last depends on how deep of a tin you use and they are "refillable."
What you need:
~Gulf wax or the wife's old candles (Gulf wax burns better and doesn't smell like mango melon ymmv)
~tins (with lids preferably. helps snuff the flames out)
~old can to melt the wax in
~markers (I used a pencil from when Neophyte was a kid, and a carpenters pencil)
~tape to seal the lids
~The 'Ove' Glove (be careful. DO NOT mess your wife's gear up)
~a big johnson or other straight edge for marking the cardboard (life is mo' bettah for those of us with big johnsons)