A study of U.K. dog owners found 23% of them leave their pets alone for five hours or more on a typical weekday, a trend that puts some dogs at risk for depression, distress and separation anxiety, some experts say. Others say the length of time dogs can safely be left alone varies by the animal and the breed. "In some cases, a dog can't cope with being alone for even 15 minutes," says veterinarian Enid Stiles. "But there are many, many other cases where (regular) dogs -- my own included -- are perfectly fine being left for eight hours a day. It becomes part of their routine."