I'm not used to RPGs where you have to do things in a certain time (ie. 150 days for the water), so should I be trying to get the chip as quickly as possible, or is it such a long time that I can take my time?
I prefer games where you can take your time, but I'm not sure if I'll be able to in this one...

As far as I know there is only the one "game ending" time limit in the patched version that is available here, and it is a lot more generous than you might think :)
I was also unsure about how quickly I had to find the chip and therefore rushed quite a bit, only to find out that there really isn't any hurry. I think I ended up having 120 days or so left :)
So, you probably shouldn't try to do each quest you come across at first and mainly pursue the goal of finding the chip, but you also shouldn't hurry too much and take your time when it is necessary.
Once you've found the water chip you still have all the time in the world to do what you want just as quickly as you want :)

You can also pay for a water caravan in The Hub, that will extend the time limit by some 30 days.

It's plenty of time considering you can travel from necropolis to vault 13 in like 4 days

actually, doing so will give you an extra 100 days to finish the main quest. careful, though, if you have completed the main quest and you talk to the water merchants about sending water to your vault, you will reset the main quest (giving you 150 + 100 days to finish it).
there's an [url=http://user.tninet.se/~jyg699a/fallout.html#v13]XP exploit[/url] related to this as well.

I wouln't worry about it too much. I did a whole butt load of side quests (kill Deathclaw, get rid of GIzmo, become a member of the Brotherhood of Steel, etc) adn I still had about fifty days left before I got the water chip

Thanks for the advice everyone!
I got the water chip today, and still had about 100 days left, so it's all good. ^_^

Ya i got the water chip, but i cant talk to the overseer?
WTH>>

you should be able to walk into the vault and talk to him directly... if not I'm not sure what to tell you.
Also, be careful about the water merchants if you arent planning on killing the super muties anytime soon, the caravans might make your vault a bit easier to find..... ... ..

Ya it was very odd..
It kept saying you cant get there.
So i dropped the water chip, then i was able to talk to the overseer,
I picked up the water chip then i was able to finish the mission.. very odd

The time limit isn't worth worrying about, for the most part.
However, if your worried about it just follow some simple tips:
Don't rest to recover HP, only when you need to wait for morning or midnight or something.
Try to finish all the quest in one place before moving on. You have plenty of time, but travel is the fastest way to use time and in most cases you only have to visit a location once or twice to finish up the quest there. (You will be in Shady Sand's 3 times, and if I remember right that's the most you need to go anyplace.)
Unless it's critical don't hire the water caravan. It dose odd things with quest timers.
The patched version of the game has a timer of 13 years. Getting the water chip still has to be done in 150 days.

I just found the water chip. Yaaaaaaaaaaaaaaaaaaaaaay!
It got me laughing; I didn't realize it was in such a simple place. Just keep on exploring the significant locations. You'll find it easily. Hint: Look everywhere. :)

one of the beauties of Fallout... you can solve major plot points by pure accident if you get lucky and just wander around enough. You do get hints from talking to people, nothing completely direct like 'here's a map' but listening to people will get you there eventually even if you don't wander in by accident...

I got to the necropolis but all the ghouls are dead and theres supermutants everywhere..whats going on?

If you wait for too long the super mutants decides to wipe out all the ghoulies I believe.