I'm not sure if anyone else is bothered by this stuff but I did the experiments and might as well share it.
Item name
Start PC time
End PC time
Total time
Fiber droped directly from inventory on the ground:
gfx/terobjs/items/flaxfibre
Time: 21:02:53
Time: 01:13:13
Hours: 4 Min: 10 Sec: 20
Flax harvested and have the fiber spawn on the ground:
gfx/terobjs/items/flaxfibre
Time: 18:59:57
Time: 23:10:05
Hours: 4 Min: 10 Sec: 7
gfx/terobjs/items/board
Time: 01:15:00
Time: 09:24:21
Hours: 8 Min: 9 Sec: 20
gfx/terobjs/items/intestines
Time: 10:56:18
Time: 11:16:38
Hours: 0 Min: 20 Sec: 20
Cow meat:
gfx/terobjs/items/meat
Time: 11:55:11
Time: 12:05:54
Hours: 0 Min: 10 Sec: 42
Sheep meat:
gfx/terobjs/items/meat
Time: 12:06:38
Time: 12:17:16
Hours: 0 Min: 10 Sec: 38
Pig meat:
gfx/terobjs/items/meat
Time: 12:32:11
Time: 12:43:02
Hours: 0 Min: 10 Sec: 50
gfx/terobjs/items/linencloth
Time: 13:02:10
Time: 21:49:06
Hours: 8 Min: 46 Sec: 55
gfx/terobjs/items/straw
Time: 22:13:22
Time: 22:34:18
Hours: 0 Min: 20 Sec: 56
My short conclusion.
First thing to note is that it seems that items dropped on the ground by hand or spawned naturally take exactly the same amount of time to despawn so all experiments were valid when dropping them directly from the inventory. The 2 different fiber despawn times shows that.
Second thing to note is that different items in the inventory that have the same resname in the world (on the ground) take the exact same amount of time to despawn no matter the type they are in your inventory. The 3 different meats Cow, Sheep and Pig meat all have the same world resname and also share the despawn time.
Boards and blocks takes ages to despawn so I guess you can easily cut them up and get to them when needed within the huge time. Same with a few other items that I haven't tested but I would assume fall into the same time category seeds and pumpkins.
The linen also has a large despawn timer (8 hours) and I think leather falls in that same category. (leather not confirmed)
Fiber from hemp and flax takes 4 hours to despawn but on the other hand it takes only 20 min for straw to despawn. I must say that its a strange thing that fiber takes 4h but straw only takes 20 minutes. It makes mega fields of flax easier to farm now with the confirmed knowledge of its long delay. But It makes me think that the straw might have been a over site by Jorb when coding there despawn timer.
Intestines takes 20 min and meats of different kinds takes 10 min to despawn. I guess this means you really don't have much time to pick up the meat when butchering and having the mats drop on the ground. Also might be good to focus on picking up the meats before picking up the intests.