the diy system can last from a week to a month depending on the amount of yeast,sugar,water,bicarbonate & temp you add in. if the recipe is right it could last up to a month.i made one for myself before and it costed me no more than $2.
1)2l bottle - foc
2)airline tubing - foc(i arld have it)
3)valve & connectors -spent 60cents
4)silicon - foc (i already had that too.)
and the solution
5)yeast -$1+ @ ntuc
6)sugar - had that at home
7)bicab. - had that lying a home
8)water - 0.0001cents?