Is there a formula to how fast something will sell out??
It would be nice to somehow stack most items to sell out at the same time or create some pattern for max income.
The Tiny Tower Wiki explains that each store has it's own demand (assuming demand is at 100% for that floor category), so sales at any given store are at a constant rate of 10 customers per minute.
Bitizen buy items randomly, so they'll buy a mix of all items and all items' stock will decrease at the same rate regardless of price. Naturally items with shorter stock will sell out faster.
Due to how sales per minute is handled, you can also "watch" your tower's stock by watching Sales Per Minute; SPM actually calculates money earned per minute, so if your SPM is high, that means bitizens are buying more high priced items. This actually means that your low cost items are out of stock, and thus your bitizens can only purchase higher cost items.
From the Wiki, about Sales Per Minute:
The wiki also has an advanced Selective Stocking method you could use:
Their method shows stocking only 2 and 3 coin items is most profitable because it yields a very high Coins Per Customer rate but doesn't close down the store as often as stocking only 3 coin items would. Note that this only works out if you're actively playing at all times, any time where the game will be left alone you should just fully stock the stores.