We brought one of these cups home from work the other day. After we noticed they'd switched from styrofoam cups to what looked like plastic in the cafeteria, we were a bit concerned about the company's green intentions. But upon closer inspection, we realized these puppies were compostable.
That's not to say they weren't ending up in plastic-lined trash bins at work, tossed in by oblivious colleagues. But we dutifully packed ours in the messenger bag and brought it home to ogle some more.
It still feels weird to toss this stuff into the compost pile—after all, it looks and feels just like petroleum-based plastic—so we did some research. According to Ecoproducts, cornstarch-based PLA (polylactide) breaks down at about 110 degrees Fahrenheit.
In commercial composting environments that bring on the heat, PLA can compost in as little as a month and a half. In your home compost heap, though, where temperatures may not get as hot as quickly, it can take a little longer. And, if you don't have a compost bin but you just toss it out into the wooded area behind your apartment like we're wont to do, it will take considerably longer to decompose on its own. Any of these options is still better than tossing it into a lined landfill.
So go ahead—throw it in and wait (patiently) for it to decompose into organic compounds.