Well...CO2 is vital for all plants to survive. But that's another science lesson. ;o)

DIY CO2 systems can last anywhere from 2 weeks to a month depending on how it's done and room conditions. Mine usually last a little over a month before having to recharge. By recharge, I mean you mix up another batch of yeast mixture.

What type of filter do you have? If you have a canister, you can put the CO2 tube right at the canisters intake. That will provide for the best diffusion. If it is an HOB kind, you can diffuse through either limewood or even a glass diffuser. Depending on you vessel of course.

As for plants...the sky's the limit. Your LFS will only carry a very select few if that. I would suggest checking online as there are a few good places out there to order from.