This whole thing with turning CO2 on and off was started by the Japanese. There are reasons that have to do with the way the plants absorb the CO2 (Diana explained it already), but also there is a concern for the biofilter, and matching the light intensity.
The biofilter does not function very well at pH values that we love to have. So at night it gets a break by not getting hit with both CO2 from the plants and from our rigged up supply.
The Japanese also time the CO2 increase and decrease with the staged light intensity that they use - the most CO2 is present when the strongest lights are on.
How much earlier you should turn on/off the CO2 is for you to find out. For example in one of my tanks it took 30 min for the CO2 to fizz out (I ran an air stone as soon as the lights went off) and 3.5 hours to build back up.
And by the way the 30 ppm CO2 is not written in stone. The 30 is suggested by EI because that approach relies on always having more than "just enough" nutrients. As many other things - the amount of a specific nutrient is specific for the method that you use and for the stage the planted tank is in at the moment. The Japanese normally maintain CO2 of 15 to 22 or so from what I can tell. Is that the max. value throughout the day I do not know.