How long does paper take to decompose?

Paper in a warm moist landfill will decompose in 2 or 3 weeks. If it is cooler and drier, then it will take longer. Paper will survive in a very dry climate for many hundreds of years (people find old newspapers in the walls of their houses).

Stronger paper, like cartridge paper and board will take longer, depending on the thickness and toughness, as well as the moisture and heat mentioned above.
It depends on dampness, proximity to soil bacteria, quantity of paper e.g. a thick book or a few pages, if shredded or not etc.

Shredded paper scattered in a garden soil disappears in a few weeks.
paper takes about 2-5 weeks to decompose.
Paper will decompose in a warm damp environment in a few weeks. In a dry environment, paper can remain intact for years.
Since it takes several months for wood to biodegrade, paper will only take about a month to biodegrade, because it is made from wood, but it isn't very thick.