Neither AA, DL, nor UA will any longer provide exact determinations of reimbursement. That is because there is no way of determining what is reasonably necessary at the time the bag does not show up and because circumstances may differ.

As a rule of thumb, do not spend more than you would spend if you knew that you would not be reimbursed. That keeps things in perspective. In addition, given that generous carry-on allowances mean that one ought to have at least the first 2-3 days of clothing in hand, what is reasonable? Are you headed home where your replacement needs are likely minimal? To a beach holiday? To a set of formal meetings?

What happens if the carrier believes that it will have the bag to you in 8-10 hours and it becomes 8-10 days?

All of these are variables unknown at the time of the report.

OP frankly wasted his own time. Why file anything until one has the bag in hand? At that point, the expenses are fixed, one has receipts and one can confirm that there is no damage to the bag itself.

While there are some carriers at some stations which distribute a prepaid card loaded with a smallish amount to help tide one over, that is only useful if one is really strapped for cash.