Well, a 2 liter bottle generator generally lasts around 10-14 days, depending on the recipe you use.
If you shake it after 8 days or so, and not as many bubbles are coming out, then it's getting ready for a recharge.
Also, you should monitor your pH levels closely as the co2 starts to roll. The pH will drop, and then stabilize as your co2 saturates the water. When the pH starts to rise again, you're running low.
Most reliable method is the drop checker though.
Maybe we can get some more seasoned experts in here to give some advice though.... i'm pretty new to the diy co2 as well!