They definitely are not outdoor dogs (like most toy breeds) -- very much an indoor dog that loves being outside but that is with the companionship of the owner. Many people leave their dog in the kitchen though you need to be sure cupboards cannot be opened as many food items are poisonous to dogs, including raisins, grapes, chocolate (especially dark chocolate, cooking choc or cocoa powder -- a quite small amount will kill a cavalier); macadamia nuts, etc.
I keep mine in an upstairs bedroom when I am out. They clearly mostly sleep though they have a couple of safe (eg nonbreakable) toys to play with. My neighbours say they rarely bark except at cats out on the roof!). I leave them water and I always give them a tripe stick to chew -- if I take the tripe out they both run upstairs as they know they get this very spoecial treat ONLY when I am going out, and in 'their' room.
If you are gone all day you need to find things to keep her busy when no one is there. Kong toys are good for this (espeically frozen ones), leave the radio on, etc. At five months a dog generally finds it very hard to hold its urine/poop for longer than an hour per month of age so she really should not be left alone longer than about 4-5 hours maxiumum. In general this should be the max time any dog is ever alone during the day. Many people either come home at lunch to give a walk and toilet break, or pay someone to walk the dog, or leave the dog at 'day care' with someone.
A companion animal goes a long long way to help relieve the boredom and loneliness of being home alone. It doesn;t have to be another cavalier of course though that is ideal!
But in general a single dog home alone all day is not a very interesting life for a dog unless you work to make it so for them as per advice above. This is of course the reality many of us face because we DO work -- but I work with a lot of dog rescue groups and in general, all strongly advise getting two dogs if you have a household where no one is home all day long. I know my two are so much more content to have a companion and they are actually *easier* to care for because the focus of the dog's life isn;t solely on you. As cavaliers bond so closely annyway I think it is healthier for them to have another point of focus rather than only the owner as they are more prone to separation anxiety (IMHO).
Oh yes to conclude I totally agree with Heather's warning -- they are at high risk of being stolen too if left outside. A lot of people like this breed and they are easy to snatch. I'd never leave a purebred of any breed outside because they offer a target for the unscrupulous, often people who want to breed from them.