I used to be diligent in making sure our personal item, our under the seat bag roller bags, each had two tops, a pair of pants or shorts depending on where we were going, a, two pair of underwear and socks and in mine a bra. Depending on where we were going I included a swimsuit and sandals. Also our toiletries, vitamins, any medicines and for me makeup and jewelry. That way if our checked bag ever got lost we would be fine thanks to the units having the washer and dryer without having to immediately run right out and shop. My husband would frequently remind me that because he needed no makeup and very few toiletries that he could fit in another t-shirt and a pair of lightweight coach's shorts or pants and be done with it for the entire vacation. So could I if I absolutely had to but unlike him I wouldn't be exceedingly happy to do just that. I've gotten lazy about taking the clothes, underwear and swimsuits. I guess the difference is that now we are retired so I wouldn't begrudge having to waste precious vacation time to go shopping if our bag got lost or delayed. Although after the last time I was shopping and came home empty handed because I didn't see anything I liked enough to buy I should give some serious thought to going back to the way I used to pack.
Knock on wood we've never had a bag lost or even delayed. Do they make you wait a few days to see if your bag turns up before they determine that it is in fact lost and compensate you?
After my bag was delayed and I had to buy new everything dh came up with what he thought was a brilliant idea. We bring a duffel that has a couple days stuff for both of us. Then if a bag is delayed we'll still have enough to get by. I reminded him he always checks the duffel as well. His response was, "well they won't lose all of the bags".
About the begrudging time to go shopping, we were only in Quebec City for a few days. Besides the time it was hard to find places in the old city that had affordable clothes that I liked. I did find Simon's department store and that is where I had the best luck.
As to your question about how long you have to wait before your luggage is declared lost, I found this:
At
a certain point, if an airline
can't locate your
bag, its status will change from
delayed to
lost. For domestic flights,
the amount of time varies by airline, though it shouldn't be more than
a few weeks. For international flights,
luggage must be
declared missing if it doesn't show up within 21 days.