I've looked through the first five pages of Scholar search results to my request "landfill decomposition of textiles cotton" and found zero useful information. Is there any scientific research at all on how long does it take for different types of textiles to decompose in a landfill?
You are correct that it does seem difficult to find information on this topic, but I found a couple resources which may be of use.
From "Textile waste resource recovery: A case study of New York State’s textile recycling system" (emphasis added):
When looking at the relative biodegradability of non-durable goods in landfills, there are three categories: Labile materials which are easily compostable and degrade fairly rapidly in about 5-10 years such as food scraps and other organic wastes; Resistant materials that are moderately degradable over 15-20 years; and Recalcitrant materials that degrade very slowly over 30-40 years in the landfill. Other outlying materials including plastics and metals hold their own label as Non-biodegradable. Within this classification system textiles are classified as Recalcitrant. However, since the mid-twentieth century there has been a rise in the use of man made synthetic fibers created from oil-derived polymers. This means that many of the textiles that end up in landfills today possess some of the same properties as plastics and will therefore never degrade.
Unfortunately the reference for this information (Black 2008) is not included in the list of references, so that's a dead end. I tried contacting the author -- I will update if they get back to me.
Another potential resource is the chapter "Biodegradation Studies of Textiles and Clothing Products" from the book Roadmap to Sustainable Textiles and Clothing. Unfortunately this reference is behind a paywall, but based on the list of references it could prove to be a valuable resource.
Landfill conditions are notoriously anaerobic, so ANYTHING in a landfill will not decompose barely at all. It may take 20-200 years or thousands. Fact is, few things decompose in landfills, organic waste anaerobically decomposes producing some methane and some residual organic hydrocarbons but plastics, last practically forever in an anaerobic environment. Better to reuse textiles wherever possible.
- Cotton: as rags. I use old t-shirts as cleaning rages, when when no longer any good, I wash em one final time then recycle cotton. Better long term strategy is that cotton can be shredded and composted to feed mushrooms. Indoor cultivation of Volvariella volvacea otherwise known as Straw Mushrooms which are edible using shredded cotton fiber. As an organic material cotton can be safely burned and used ash as a source of fertilizer, compost amendment.
- Fleece is made of polyester. Soda bottles, which can be sent to plastic recycling to make, more fleece or soda bottles.
The overall strategy is mitigate sending textiles to landfill in the first place. Donate clothes to charitable donations groups. Cotton (100%) can be bonfire fuel. Synthetic fabrics, should be recycled.