My lovely Estrela Mountain Dog is a year old now.
I leave her alone at home for 4.5 hours, 5 days a week to go to work.

The longest period I have ever left her by herself is 6 hours, and I've only done that a couple of times.
She's been totally housetrained for over six months. 

How long is the longest I can leave her for?

I would never normally do this but we have got four day passes for Crufts because she has been chosen to do Discover Dogs on the Sunday :) :) :) :) (I am so excited!!!)  I want to take the family on Saturday to have a look round but it will take us at least an hour each way travelling.  Thanks :)

You're very lucky to be able to leave your dog. I am fortunate in this area too.
If it were me i wouldn't want to leave her any longer than you do already. If you are worried you are out all day could you not give your key to someone (trusted friend or family) and ask them to pop in.
Alternatively have you got a sensible young niece/nephew/friends child of teenage age? When i was about 14 my aunty and uncle used to buy a load of treats in and i'd stay at their house for a good part of the day with their dog if they were out for a greeat length of time. It was win win really. my parents knew i was safe and had the phone with me if there was any bother, the dog got company and looked after, aunty and uncle knew the dog was in safe hands and wouldn't get bored or ruin the house, and i got a few hours on my own to watch tv/do homework etc.

Yes I would try and get someone to come in and check on her, you will enjoy the day more if you are not constantly worried about her being left alone.
Hope you enjoy yourself on both days :-)