How low should a company let its inventory drop before it places an order? (i.e. How many units should be in the inventory at the time a new order is placed?)

The answer is highly situation-specific. A "safety stock" or "base stock" model takes as its inputs such factors as your uncertainty of demand, estimated costs of stockouts, inventory perishability, and your inventory carrying costs. All of these factors are, of course, specific to each individual business.

The calculations for one company might suggest it should try to go no lower than having 7 days' worth of sales on hand, whereas another company might need to hold no less than 25 days' worth of sales, and no more than 40 days.

See here (http://office.microsoft.com/en-us/excel-help/determining-the-reorder-point-how-low-should-i-let-my-inventory-level-go-before-i-reorder-HA001124602.aspx) for an example of using a spreadsheet's built-in statistical functions to calculate an Economic Order Quantity (EOQ) model where demand is uncertain.

Since you posted this under Homework Help, I assume this was a homework question and not a real life situation. As your first response indicated, there is no general rule here. It depends on the company, what their consumables are, what their sales are, what their storage space is and other factors.

But since this is a homework question YOU have to provide an answer based on the scenario given for the homework. So, if you want our help let us know the work you have done and we can critique it.