Answer: If you placed these materials in a large pile of at least three feet high and wide and deep, and kept it moist and turned it regularly, you might have usable finished or nearly finished compost in two months -- assuming you were doing this during the summer months. In cooler weather the process takes longer and in cold weather it basically stops when the material freezes.
If you sheet composted these materials by spreading them over the soil surface, it would probably take all summer for the materials to begin to break down. It would be much faster if you turned them under the soil surface to expose them to more active decomposing action.
If you have a garden area with bare soil, you can use the leaves and grass clippings and shredded paper as a mulch. (Mulch is essentially sheet composting, the mulch breaks down continuously and must be replenished periodically.) That way you could plant now.
If you are trying to smother an area of lawn in preparation for planting, it will probably take all summer and you can plant in the fall. To make it work faster, you could cover the area with damp newspaper and pile the grass/leaves on top. Leave this until the grass is smothered to death in a few weeks. Then spade the dead grass and other material into the soil.
In my experience, fruit/vegetable food scraps (no meat, dairy or fats) need to be buried or composted or you will attract (undesirable) animals.
I hope this answers your question.
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