Three beagles, three different set of rules.
Chipper, our first, we could leave along after he was about a year old. But we paid the price before that in a new couch, a new recliner, countless shoes and slippers, sock, underwear, stuffed animals and opened cupboard.
Cobi, we still have to make sure that the doors are shut, and refrigerator is tied tightly...he is 7 but behaves like a puppy sometimes. If he is having a bad day, he sleeps in his kennel while we are gone
Casie, the baby at 18 months --- is kenneled everytime we leave the house. He doesn't mind it, he was in a kennel most of his puppy life without us. He absolutely can't be trusted yet.
How long do we leave them, well, that depends...we let them out before we go, and then count three hours before we come back or arrange for someone to let them out if it's longer.
They have all slept with us, including Casie, who still has the need to go out in the middle of the night, since they were trained. Until then, we had smaller sleeping crates in the bedroom walk-in closet, near the bed....I can't imagine not knowing what they are doing letting them run the house at night...that's an insurance man's nightmare.