I am referring to the actual items lying around on tabletops and shelves in any store.

For example, the ingredients and books on the counter in Arcadia's Cauldron in Whiterun.

Do they respawn? if so, how long is it? is it in-game time, or real time?

*I have tried setting the timescale to 500,000 and standing outside to pass time as quickly as possible. After about 20 days I re-entered the store only to find empty shelves and counters.

I have also wondered this. I've never actually tested to find out however.

They do respawn, but I have no idea as to when it is. I take the ingredients from Arcadia's Cauldron all the time.

UPDATE:

I just tested it for a longer period of time.

I stood right outside of the store and set the timescale to 1,000,000.
A few months passed.

Went back into the store, and all the ingredients and books reappeared on the counters and shelves.

I don't steal from shops, it's just a rule of mine, but I knock the bow off of the Drunken Huntsman's counter every time I buy arrows. It always goes back.

It would be nice if the merchants stuff came back, as the reason I developed this rule is one of my characters robbed Balmora blind and for the rest of the game, I went into empty stores. It was weird buying from Merchants who had nothing.

I only take stuff from friends houses w/c i also hope respawn like Hod and Gerdur's house

I give Arcadia the frost salts, so technically I'm not stealing anything.

My thief took everything from the Alchemy shop in Whiterun and it eventually respawned but it did take quite a while. For a long time I thought it wasn't coming back but eventually it did. No idea what the time frame is though.

I just tested.
It seems it's one month.

Thanks. OP wasn't the only one who wanted to know.