Separation-related distress and hot-weather risk are different problems, but they can overlap when a dog is home alone. A cooling-system failure can become a safety problem regardless of whether a dog has separation anxiety.
There is no universal “safe maximum” number of hours that applies to every dog. Age, bladder needs, medical conditions, behavior, access to water, the reliability of the home environment and the individual dog all matter.
Persistent vocalizing, destructive escape attempts, pacing, drooling, house-soiling or panic around departures can warrant a conversation with a veterinarian or qualified behavior professional. A camera can help you observe what happens after you leave.
The Dog Alone Time Calculator can help you think through age and routine, but its output is not a medical or safety clearance. If the home may lose cooling, a pet has health problems, or the dog shows distress, shorten the time alone and arrange a check-in or other care.
If a dog appears overheated, weak, confused, collapses, has trouble breathing, or otherwise seems acutely unwell, seek veterinary help promptly.