Not sure about your filter type and such but it should be receiving oxygen instead of c02 for the bacteria to live, therefore hooking the c02 up to the intake of the filter is not the best choice, in fact I would hook it up to the output returning the water flow down into the tank directionally so that the c02 will be in contact with the water column longer.
As for how long it will last there are variables like water chemistry (mainly Ph from tap) how many bubbles per second are the main ones. You'll know how long after the first bottle is used up and you create your second bottle the same way you can expect the same amount of time then. Here's a link for complete info on the subject especially about the amounts of each ingredient used in the bottle, and there effects if reduced/increased. http://www.qsl.net/w2wdx/aquaria/diyco2.html#6