Answer:
Plastic will degrade at a varying rate depending on it's quality. From days, to years.
Now, by degrade we mean it will break apart into smaller pieces of plastic, right down to single molecules. However, if you're meaning break down into different substances which could possibly be of use in nature (such as living things do) then the answer is never. Every molecule of plastic we have made which we haven't manually changed to a different form is still in nature.
This fact is actually a very critical environmental concern currently, as there is a massive amount of these single molecules of plastic in the oceans which are effecting fish populations (and things which eat fish, like us)