I've occasionally had to leave an item, like wood or stones, uncollected on the ground due to a full inventory. I notice that they persist between area changes (so I can cut down a tree in the Backwoods, run home and clear out my inventory, then go collect the remaining items), and I once picked up some hardwood I'd forgotten about a day or two after cutting it.
How long to items persist outside on the ground? They persist at least a day, but for how long in general: a set number of days, the whole season, … or indefinitely?