Dogs actually smell the passage of time, because of their super sniffing powers. They have 50x more receptor cells than a human nose and can therefore tell how long ago you left the house, by how much of your scent is still left in their airspace. The longer you've been away the weaker the smell becomes. so time actually smells different to dogs throughout the entire day; for example 3pm smells different to 6pm, which can explain why they are so excited to see you when you get in from work.
Some dogs are very energetic and require the need for attention or exercise during the time you're not present and of course will need to relieve themselves at some point. Certain dogs can become disruptive, particularly if they chew and bark. For the well-being of your dog, you should not leave them alone too long. Adult dogs (over 18 months) can be left alone four to five hours a day (max), whereas for puppies two hours a day is the absolute limit!