Plastics take decades centuries to degrade, and most of that degradation is physical - it gets degraded to microplastics. As soon as it's in a landfill where it's not in motion or exposed to sunlight, it's essentially conserved for human lifetime cycles.
Until too much water makes it into the landfill, creates an acid with other mixed materials (like cardboard glues, household cleaners) and then leaches the acids and plastic and heavy metal into the aquifer.
Most landfills near me with liners made in the last 30 years have acid problems and liner breaches