So, for example, if and item had a durability of average and a tt value 20 ped, will it last longer or shorter than an item with an item with the same durability of average but a tt value of 100 ped, or is there another element involved?
Use it once then put it in the sell section of the trade terminal (But for the love of god dont press sell) then add 1 bottle of sweat at a time until the sell price is equal to the full TT of the item.
The sweat is worth 1/1000th of a pec each so you can then work out based on how much sweat had to be added exactly (or at least to the nearest 1/1000th of a pec) how much the item decays per single use.
PS sweat/dung/fruit are all worth 1/1000th of a pec at the TT so any of these can be used for the test.
The exact decay per use of every item is not stated ingame anywhere. It's just aluded to via that durability description you mentioned.
However, with a lot of work (and some standardized testing methods developed by the players) we've been able to work out the exact decay of about 90-95% of all ingame items.
And generally, the items without decay stats aer so rare, you'll never touch them anyway, so no real loss there.
--moderator's note--
Moved to Items section.
--/moderator's note--