Most dog owners go out to work, shop or see friends, so the question is never whether a dog will be alone, but for how long and how ready they are. The PDSA and the RSPCA both suggest that adult dogs should not be left alone for more than about four hours at a time. That gives them room for exercise, a toilet break and company.
Four hours is a ceiling, not a target. Every dog is different. Very young puppies need less, and some dogs are upset after just a few minutes because nobody ever taught them that being alone is safe. That teaching is a skill, and it is built slowly.
Dogs are social animals. Being alone is something they learn to be calm about, rather than something they simply know. Both the PDSA and the RSPCA describe the same kind-and-slow approach. Practise each rung until your dog is relaxed, then move up. If your dog gets worried, go back one rung. That is not failing, it is how the method works.
The golden rule: always stay under the point where your dog starts to get worried.
A tired, comfortable dog finds it easier to settle. The RSPCA suggests a walk that finishes about half an hour before you go, a small meal, and a toilet break. Offer something safe to do, such as a stuffed chew toy or a treat puzzle, and take it away when you return. Make leaving and coming home calm and low-key rather than a big event, so neither feels dramatic.
Never shout at or punish a dog for a mess, chewing or barking that happened while you were out. They cannot link it to what they did hours ago, and being scared makes worry worse. If your dog is already worried, long stretches alone can make it worse, so arrange cover while you train.
The RSPCA lists barking, howling, toileting indoors and chewing or other destructive behaviour. Quieter signs are trembling, pacing and whining. The RSPCA suggests filming your dog while you are out, because some dogs look calm to the neighbours but are distressed. Neighbour complaints about noise can be a clue too.
If you see these signs, talk to your vet first. The PDSA advises that pain or a medical problem should be ruled out, and a vet can refer you to an accredited behaviourist if needed. Severe cases need professional help, not just home practice. Please do not try to diagnose it yourself.
Advice differs a little between organisations and countries, and every dog is an individual. If you are worried about your dog, ask your vet.