I don't know of data on bagged-leaf decomposition in landfill settings, although there may be some somewhere. One place to look for serious municipal composting information is the composting operation of Halifax, Nova Scotia. Leave it to the Scots and Canadians in general to implement a unique combination of good policy, citizen support and participation, intelligence, and good sense. Probably we can infer the answer to your question from the general state of organic matter decomposition in landfills. The decomposition time likely would vary considerably depending on how wet the leaves are when they enter the landfill and whether there is an associated nitrogen source. It seems to me that fairly dry leaves would last years, probably decades, in a landfill. It might matter some if the bags remain intact and sealed, but the bags probably break some when the landfill cell is compacted and covered.
The landfill environment strives to keep the material dry, anoxic, dark, and at a cool and stable temperature. Under the low or no-oxygen environment, the decomposition would be anaerobic, so it really isn't considered to be a composting process. That process tends to generate methane and hydrogen sulfide gas. The landfill process sort of becomes a Catch-22--robust decomposition would break the plastics, likely including the plastic landfill liners, down over a shorter time into small units of toxic material. Eventually, the groundwater would be (more) affected than it already is. Landfills surely are one of the worst places to put leaves, although burying the carbon does remove it from the atmospheric cycle, at least temporarily. One of the worst justifications I've ever seen for landfilling plant material is to increase the tipping feed that the landfill receives/generates.
Aerobic composting, on the other hand, breaks down the leaf material in a comparatively short time--weeks to months--into the fine and useful material good gardeners know so well. If composting is too much trouble, even the process of mulching the leaves into the turf uses substantially less enegy than the bagging/landfilling process and yields many of the benefits of composting. It is beyond me why people want to waste the money and time to remove fertilizer from their lawns and gardens, but ignorance surely plays in that calculus somewhere. I know of one city, which picks up yard waste only if it is placed in special paper bags. Then, the bags and plant material are shredded together, composted, and the compost is made available free to the residents. If yard waste must be collected to satisfy a psychotic electorate, that seems to me to be one of the better ways to do it.
In theory, theory and practice are the same; in practice, they aren't -- lament of the synthetic lifestyle.