You can recover half of the expended ammunition, rounded down.
Page 146 in the Players Handbook is the only source I can find that mentions anything regarding arrows being destroyed or used up, in general:
Many weapons have special properties related
to their use, as shown in the Weapons table. Ammunition. You can use a
weapon that has the ammunition property to make a ranged attack only
if you have ammunition to fire from the weapon. Each time you attack
with the weapon, you expend one piece of ammunition. Drawing the
ammunition from a quiver, case, or other container is part of the
attack. At the end of the battle, you can recover half your expended
ammunition by taking a minute to search the battlefield.
We combine this rule with the “Round down” rule from the introduction to the PHB:
There’s one more general rule you need to know at the outset. Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.
So we have the rule for ammunition recovery: at the end of battle, you may recover half of the expended ammunition, rounded down to a whole number.
For example, if we fire 7 arrows during the fight, we can recover half of 7, which is 3.5, rounded down, which is 3 arrow recovered. If we only fire a single arrow, we can recover half of 1, which 0.5, rounded down, which is 0 arrows recovered.
As for arrows disappearing and being used up because of wear and tear, I see no actual rules for this. You could assume that half the arrows you don't find were arrows that broke off during the fight, or were too damaged to reuse.
The answer to your questions, in order of your:
- "Is ammo reusable and recoverable" Yes
- "Is there any guidance on how long an arrow/bolt/dart should last,
- "In the absence of RAW guidance, can someone suggest a mechanism for
ammunition recovery...." Other than using the RAW, as shown, it might be
easier to talk to your players and see if they would like to do this