Imagine yourself in a big mall. There are no zombie around you. Yet...

You take some tools and find air vent shaft, take off the door crawl in and put the door back and nail it with hammer. There are no other ways inside the vent shafts, and there is enough space to live. You took lots of supplies, found mini fridge or w/e to keep your food and water fresh. Can zombies smell you? Theyre in the mall now and theyre wandering around. First, they are dumb, I dont think they know how to use tools and they cant break into. Possible?

Well if we are talking "zombies" as in the Romero style then no probably not gonna find you. If you do make noise or give away your position enough of them may be able to break through your fortified vent, unless it is elevated beyond their reach. The biggest problem is can you deal with this long term. Cabin fever, the realization of what is going on in the world may be enough to drive you crazy in long term confinement. Most humans don't do well without interacting with other people over long periods of time anyway. I think in a zombie outbreak the biggest killer, after the first 3 months will be on a psychological level anyway. People won't be able to cope with the isolation, the constant threat, and the more than likely loss of many many loved ones. Most will just give up. On top of that, after enough time has passed I think other living people would become your biggest threat. You have supplies, they want them... They will kill you for them.

In theory tho, if you could stand the isolation and had enough supplies then yes you could survive indefinitely if you were thorough about securing the area that you are holding up. One thing I did wonder, i know it was just an example but, how would you wire in your electronic devices in a vent? Extension chords would eventually get pulled, generators would give away your location and most importantly kill you if breathed in too much of the fumes, which is likely if you have sealed yourself up... The only way around this that I can think of is to place the generator as far away from you in the vent system as possible( or on the roof considering their is a vent shaft up their) and run your chords through the vent shafts themselves. You would have to have direct access to the generator and a very large quantities of fuel stockpiled. With the generator this far away you wouldn't give away your direct position but it would still be risky.

Well, if by any chance a licker comes around or a zombie evolves into a more advanced stage (crimson head, for example) you're pretty much screwed. Especially in the licker case. Other than that plus what Boblathon said you'd propably be fine.

I was thinking about big ass vent areas, not the small ones. Well, wasnt pointing on being alone and stuff, thats other story, but lets say you are able to hide in there for, ummmm... lets say 3 months? Now, most people are zombies now, and there aint another humans to hunt so they dont have anything to eat. Is it possible that they would die or attack each other in extreme situation? Or is 3 months even enough?

Too many variables with that topic.... Does the virus slow the metabolism so far that it would take forever for the body to burn through the stored energy? Does the virus shut down the systems in the body thus the body no longer requires food as a source of energy? Does the virus mutate the cells in the body to the point that they can sustain themselves indefinitely while infected with the virus? Until they come out with some kind of detailed literature explaining the nature of the virus all we can do is speculate on the ultimate outcome of the host.