I have a page that's cached by Google in March, and the page no longer online(it's actually up for about 12 hours). However, Google is still having the cached page... So how long does Google keep it?

Good question, I have pages that I deleted months ago (or a year), but they still appear in the Google index.

Forever ???

You can remove the cached pages from Google index.

Register here http://services.google.com:8882/urlconsole/controller?cmd=reload&lastcmd=login

I had the same issue with my pages but got deleted after i posted a request. It may take 5 days to remove content from Google.

I've seen pages from Google's cache that say they were from a year or so ago. Presumably it just hadn't been modified since then, so Google didn't update their cache. However, with their recent changes to the way they display the dates on cached versions [*], I wouldn't expect to see anything in the cache with a date older than a couple of months.

[*] http://googlewebmastercentral.blogspot.com/2006/09/better-details-about-when-googlebot.html

*The page was from March and only up for about 12 hours..*