> Formation of methane and CO2 commences about six months after depositing the landfill material. The evolution of gas reaches a maximum at about 20 years, then declines over the course of decades. https://en.wikipedia.org/wiki/Landfill_gas

From the top charts for image search "landfill gas chart", suggests methane(CH4) vents heavily for 3-5 years, then a long taper.