Researchers at the University of California, Santa Barbara reviewed every published measurement of how fast common plastics break down under different conditions, and converted them into comparable degradation rates.
They estimated that a high-density polyethylene bottle would take about 58 years in the marine environment for half of it to break down (its 'half-life'), while an HDPE pipe, being much thicker, would take about 1,200 years. For PET (drink bottles) and polystyrene, surface degradation was so slow as to be effectively unmeasurable, with extrapolated half-lives above 2,500 years.
The review matters because the popular '450 years' and '1,000 years' figures for plastic have no traceable primary source. It also clarifies that 'degrade' means fragmenting into micro- and nanoplastics, not disappearing.
Recommended wording: plastics do not biodegrade on human timescales; they fragment into micro- and nanoplastics that persist for decades to centuries.