Regular testing of pH is the best way to tell what is happening with regards to CO2
in your tank.
It's hard to guess how low your pH will eventually get..... it depends on your KH value (buffering ability), CO2
loss at surface, CO2
use by Submerged Aquatic Macrophytes (SAM's), bubble rate etc.
Testing before the lights go on will tell you how much you have available for SAM's.
Testing just before/at lights out will tell you how much the SAM's have used during the day.
Testing during lights on (throughout the day) will tell you even more about your CO2
levels, but is probably unnecessary.
Testing during the night will tell you either; a) how much CO2
is building up when SAM's aren't using any (if you leave CO2
on), or b) the rate at which CO2
drops if you turn it off overnight.
I invested in a pocket pH meter which allows me to test whenever I like without having to spend time and a lot of money on liquid test kits!
A permanent pH meter (in-tank probe) is another (more expensive) option to be able to monitor pH constantly.
I would test day and night (before lights come on / just after lights go out) to monitor levels.