I managed to survive for 4 weeks, give or take a few days extra.
The key is to not only board up the windows on ground level, but also the windows upstairs. Zombies standing downstairs can still see you when you are above them (at least a single story higher then them) if your standing in front of the windows. Basic rule: if you can see them, they can see you.
A tip: if you've got a fire axe, you can chop down useless doors (like the one in Kate's bedroom leading to the bathroom) and use the wood that falls down to barricade the upper windows).
I read on the forums that a zombie banging on your door might attract attention from others. So, if you see a lonesome bugger hammering at your gates, kill him, even if he isnt doing any direct harm.
In general you might want to keep the area around your safehouse clear of walking dead, if there arent too many.
If walking into town and coming across a big horde, you might want to consider rerouting your way back to the safehouse.
Dont go in a straight line for it, because i have suspicions they will just follow the general direction they last saw you leave in. Lose them in alleys, fool them in going a different direction, then swing around and go back to the safehouse.
Also, Kate should be quite safe if you leave her alone for a night.
During my 4 week ordeal i went out scavenging, found 2 shotguns, a bunch of ammo, a lot of food, nails, batteries, and a flashlight.
I was very encumbered, and couldnt make it back to my safehouse in time. It was getting late, and to make things worse i saw the lone stranger who had tried to kill me earlier in the supermarket out on the streets with his back turned.
I had to shoot him, and i did. But being as encumbered as i was, i wasnt going to get far if i didnt want to lose all my newly acquired gear. My legs grew tired after every 20 metres or so, my character was becoming very tired and it was getting late.
So i holed up in a building for the night. Next morning i wanted to go out, and sure enough the horde was waiting for me.
I ran back inside, barricaded the doors and what followed was a two-day long siege of lack of food, lack of sleep. Finally the barricades went down, and the horde started pouring in. Thankfully i had gathered around 90 shells, and waiting at the stairs i managed to dispatch most of the horde.
After a day-long firefight (i wasnt risking getting scratched or bitten by this group, which i surely would have been in melee) i managed to get out of the buildings only exit, only to encounter even more zombies coming my way.
I had ditched my spare shotgun, and unloaded a lot of ammo during the zombie siege previously, and ate all my spare food i had collected, so i wasnt that encumbered anymore. I ran like hell, but still didnt manage to get back home.
It took me 4 days of migrating through town to get back to my safehouse, and when i got back i didnt have a single item more then when i had left 4 days earlier.
As i walked inside and barricaded the door, settling from a hefty panic, i went upstairs and was met by a mostly disappointed Kate