Simple answer: you don't know
But it's rather difficult to get high enough CO2 levels that are going to affect your tank. Keep in mind that CO2 in water does not displace O2, so increasing CO2 production isn't directly going to cause issue to your fish.
For a 29gal tank, I would run two 2L bottles, connected by a T-piece, and each one having a check valve. Standard DIY mixtures last 2-3 weeks, with the last week being slower production. By using 2 bottles, you can run each bottle for 2 wks, alternating which one is changed each week. This will give you more consistent CO2 levels, and curb algae growth due to fluctuating CO2 levels. A check valve on each allows a bottle to be removed, without losing pressure across the system.
Unless you had planned on running 6 bottles (you'd want plenty more lighting anyway), I wouldn't be concerned.