Hi,
Have a look at the 'background' tab from the project you selected:
https://www.sciencebuddies.org/science- ... background
I used some of those keywords in google and looked around a bit. It was a little hard finding some good resources, so I suggest you use sentences to search, like these ones:
Do paper towels biodegrade?
How long does it take for eggshells to biodegrade?
And basically, understand that biodegradable simply means to be consumed by microorganisms and return to compounds found in nature. Generally simple items that came from nature biodegrade well, like clumps of vegtables and grass clippings. Other food items and sturdier natural items, like eggshells or wood or paper, take a bit longer. Synthetic things like cans, glass, cement, etc. take very long times - we consider this not biodegradable. Since bacteria and fungus are doing all the work at breaking down the items, the speed this happens is related to the soil you dig in, it's composition, temperature, moisture level, etc. The wikipedia article on biodegredation is a pretty good start!