It's a matter of training and having an appropriate set up.
All of my dogs can be left on their own but you really need to know your dog very well before ever leaving them with the run of the house–and I agree, puppies should never have the run of the house as they can get into all sorts of mischief, some of it dangerous. And they should never be just left out in a back garden on their own. It is much safer generally to have dogs in an indoor, enclosed space–this may mean in the kitchen or some other room with their dog beds, some appropriate toys, and I would always leave the radio on and fresh water. The toys need to be absolutely safe where a dog cannot tear things apart and choke on pieces. Another consideration with multiple dogs, is whether they will fight over toys as this could also be very dangerous.
You can generally work to increase time, over time. For puppies, it is best not to leave them for long periods not least because it damages your house training approach and also can be very disturbing for them. As a rule of thumb, most puppies can hold themselves for one hour for every month of age, +1. So an 8 week old puppy could be left alone for 2 hours–but of course with that age, you are very likely going to have accidents anyway. With the 4-month-old, the rule of thumb would say 5 hours–but I cannot imagine a 4 month puppy that would not be having accidents in a five-hour., As it's quite long and generally you are really only beginning to get into the hang of house training. I sure would not want to leave a puppy that young alone for more than a couple of hours, say 3 to 4 maximum. If people are going to leave a puppy longer than that routinely, I don't think a puppy is a good idea–it is much better to get an adult dog, or to work to find a doggie daycare setup where'd the puppy can be left to get adequate stimulation and activity each day (but with the puppy–I just don't see the point as then the puppy would be spending almost all of its hours awake with people at a daycare center rather than with its owner, and won't be getting any house training–it's just kind of a recipe for ongoing problems). Dogs are very social, and to be left alone for hours every day is a very under stimulating life. Of course, people with full-time jobs can absolutely work around this through many different approaches, including having someone come in midday to give the dog a walk, to have the dog go to daycare a couple of days a week so at least there's a a regular weekly daytime opportunity for lots of interaction and fun, or coming home at lunch to give the dog some one-on-one time and to let them relieve themselves and so on.
I do think it is really important for everybody to have a dog that can be left alone and remain calm, self-confident and happy
. This is part of preventing the onset of separation anxiety and other problems that can make dog ownership difficult–but it does take some owner investment as dogs generally cannot learn to be left alone by just leaving them! You can get great advice on training for this and many other puppy and dog issues by downloading the free book After You Get Your Puppy at
www.dogstardaily.com/free-downloads
I have 4 cavaliers, and 2 of them are penned into the kitchen with the other 2 having the run of the house. One of the ones who is in the kitchen is a next puppy farm dog who will probably never be fully house trained and so I keep her there so that it doesn't matter if she has a little accident on the floor, and I keep another dog in there with her for company.
there is just a gate across the doorway, so they can interact with the other dogs and the cats. This system works well for me. as all of them are mature adults, they are fine being left alone for short periods to long periods. They have been on their own for 8 hours at times, but this is rare as I work from home.