If you don’t reuse or recycle that item, it probably will end up in a landfill.
Once in a landfill the only thing left for it to do is decompose.
Decomposition rates (rate at which it will totally break down into the earth) of items in landfills will vary depending on the amount of sunlight, moisture and air exposure it receives.
Some of these time ranges are very rough estimates.
The following breaks it down for you:
Apple core: 1 to 2 months, can take longer in landfills due to lack of microbes
Glass bottles: tens of thousands of years; glass is made from sand and it can outlast most anything
Plastic drinking bottles: hundreds of years; consist of polyethylene terephthalate (PET) which is made from petroleum, which won’t break down
Plastic bags: up to hundreds of years; newer plastic bags can photo-degrade, but most aren’t exposed to sunlight when in a landfill
Milk carton: 5 years
Plastic milk jug: 500 years
Aluminum can: 80 to 200 years
Styrofoam: no sign of ever breaking down
Cigarette butt: 1 to 5 years
Newspaper: 2 to 4 weeks, can take longer in landfills due to lack of microbes; will decompose much faster when wet
Sources: “Pocket Guide to Marine Debris” from The Ocean Conservancy (2004), How Long Does It Take Waste Materials to Decompose and “Garbage In, Garbage Out,” Audubon magazine, Sept/Oct 1998.