There are several factors that determine if an item will despawn when left on the ground:

Is the item able to be protected by the flag? Only certain items are protected. ONLY items that show the flag symbol when you look at them will be protected by the flag.

Is the item within the flag radius, which is 60m? Items that are able to be protected by the flag will show the flag symbol IF they are within 60m of the flag.

All items have their own lifespan defined by the server configuration. This is the lifespan they default to if they are not protected by the flag. Some items such as crates, sea chests, barrels, fences, lockers and many others have a lifespan of 45 days.

Other items have various lifespans of their own ranging anywhere from minutes to hours. Your best bet is to assume that anything you drop will despawn when you leave the area unless it is a flag protected item.