I have a small 60x30x35cm tank with 8 potted plants. I was wondering

how much CO2 would my plants need? Currently, I'am using the commonly

used DIY CO2 reactor...

If your co2 is bubbling on a regular basis, that should be enough for a small tank.

Are your plants growing well, and do you ever see them pearling during the day?

The optimal range of CO2 for plants is 20-30ppm.
You can figure out how much CO2 is dissolved if you know your KH and pH readings. Here's a calculator I built for that: http://dataguru.org/misc/aquarium/calCO2.asp

Here's a table that gives you the same info:
http://www.floridadriftwood.com/page.asp?id=16

In a planted tank with lots of light and photosynthesis going on, you'll have pH swings daily. pH will be the highest right before you turn off the lights, as the plants are using up CO2. pH will be the lowest in the morning right before you turn on the lights cuz without light, plants release CO2 into the water. I don't know how much adding DIY will affect that swing. I'd bet it'd cause a downward shift in pH from what you'd see without DIY CO2.

Yep, what DataGuru said. You have to calculate the amount using pH and KH.

I think one of the main problems with using DIY C02 is that it isn't consistent. When you first hook the bottle up, it bubbles like crazy, then it peters out, so at the end of its life, it is producing a lot less CO2.

I have 4 watts of light per gallon, no added CO2, no additional ferts but rich substrate, some surface agitation happening and all my plants are growing like crazy. I do think I'm getting more CO2 with the plant filter being on an opposite light schedule from the goldie tank. Also, since I'm using the topsoil substrate, the plants should be able to pull a lot of their carbon from there.

Describe your set up for your plant tank a little more. It sounds interesting

Here ya go. I just built a web page yesterday describing it:
http://dataguru.org/misc/aquarium/plantfilter.html