The residence times of plastics in the oceans are unknown, largely because of the durability of the material and the relatively short (decadal) period of time over which plastic products have been manufactured. 

In this study conducted by Andrew Turner and colleagues, classic LEGO bricks constructed of acrylonitrile butadiene styrene (ABS) and washed up on beaches of southwest England have been subjected to X-ray fluorescence (XRF) analysis and the spectra and any other identifiers matched with unweathered blocks stored in collections or sets of known history. Relative to unweathered equivalents, weathered blocks exhibit varying degrees of yellowing, fracturing and fouling, and are of lower mass, average stud height and mechanical strength. These effects are attributed to photo-oxidative degradation and the actions of physical stress and abrasion while exposed to the marine environment. Infrared spectra indicate that the polymer remains largely intact on weathering but with photo-degradation of the polybutadiene phase of ABS, while quantification of XRF spectra reveals that pigments like cadmium sulphoselenide become more heterogeneously distributed in the matrix when in the environment. 

Using measured mass loss of paired (weathered versus unweathered) equivalents and the age of blocks obtained from storage the study estimates residence times of between about 100 and 1300 years for this type and thickness of plastic, with variations reflecting differences in precise additive composition and modes of weathering.

Weathering and persistence of plastic in the marine environment: Lessons from LEGO