it lasts untill the bubbles dont show up when shaken. it does smell like fuckin bread or bear. its icky, it got too stinky for me, i dont do that anymore.
a good way is to get 2 of those plastic party cups, 2 sizes. one that will fit in the other one.
take the smaller one, put some vinigar in it, about 1/6 of the way full. then poke a hole in the bigger one, poke a string through the hole so it dangles in the vinigar when you put the bottom of the bigger cup in the top of the smaller one. make sure there is a coil of string on the inside of the big cup and poor some baking soda in the bottom so it covers the string. put some plastic wrap over the top of the big cup so powder doesnt get blown anywere, poke holes in the plastic.
this is basicaly a wick system, the string will carry vinigar from the bottom cup to the baking powder in the top, when vinigar comes in contact with baking powder it releases co2.