Typically when plastic ends up in a landfill, if the plastic is no longer exposed to sunlight, it could be a very long time before any decomposition occurs with buried plastic. Alternately plastic exposed to direct sunlight can decompose more rapidly which creates a problem with plastic pollution in oceans, lakes, and plastic thrown on the ground outside.

The Guardian reported on a study from Orb Media finding 94% of samples from the study taken in the United States including samples from Congress buildings, and Trump Tower located in New York had plastic fibers in the tap water.

You are able to help with global plastic pollution and microplastic contamination, simply by reducing, or eliminating, the amount of plastic bottled water you buy and use. Use a water filtration system at home instead of buying bottled water and fill up a reusable water bottle.

Sources:

https://www.theguardian.com/environment/2017/sep/06/plastic-fibres-found-tap-water-around-world-study-reveals

http://science.howstuffworks.com/science-vs-myth/everyday-myths/how-long-does-it-take-for-plastics-to-biodegrade.htm

http://www.deccanchronicle.com/nation/current-affairs/140917/micro-plastic-fibres-found-in-tap-water.html