How long will an un-tapped keg last?

New guy here, curious about kegging my home brew. I assume that once the brew is put into the keg, it has to be carbonated with CO2 immediately? Also, how long will the keg last before the beer starts to ho bad? Does it last longer in kegs rather than bottles?

Depends on the beer. Ipa's should be enjoyed sooner than later. Lagers can take months to age out.

It doesn't necessarily need to be "carbonated" right away, you just want to make sure and purge the regular air out with CO2. If I know I'm going to let a beer sit for a while, I just purge the oxygen on kegging day, then maybe hit it with some CO2 every couple of days, when I think about it. Really the only thing you need to worry about is getting any oxygen away from the beer.

I usually don't bother worrying about actually carbonating the beer until it goes into the kegerator.

As Rockn said though, if it's an IPA, don't let it sit too long. It's not about spoilage, but losing that upfront hoppiness that you want in an IPA.