The Montreal Convention (1999) which applies to international air travel, states the following in relation to lost luggage:
“If the carrier admits to the loss of checked luggage, or if the checked luggage has not arrived at the expiration of 21 days after the date on which it ought to have arrived, the passenger is entitled to enforce against the carrier the rights which flow from the contract of carriage”
Most reputable airlines will supply at least a toilet bag and reasonable costs if you’re luggage is delayed.
Friends I was skiing with had their luggage delayed for two days after flying with British Airways from Heathrow to Vancouver. As the purpose of their holiday was to go skiing and as they didn’t have any of their luggage they never had the necessary clothing for their holiday. They purchased the necessary clothing and successfully recouped the costs from British Airways. I think this was more a goodwill gesture on the part of British Airways than obligatory as it appears from the terms of the Montreal Convention that you’re not entitled to any compensation for delayed luggage, just compensation from lost luggage after 21 days.