(please gently redirect me if it's a wrong SE and don't question-ban me, I mean no harm)

Paper products have a lag time of 1.5–2.0 years before methane production, a ‘half-life’ of 10–20 years, [...]

(from here, p. 150)

Does it mean that it takes 20–40 years for a piece of paper or cardboard to fully decompose in a landfill? Apparently, not, the two halves are not identical. What is a "full-life" then? Can it be calculated with a "half-life" estimate at hand?