actualizations

Usually older files get replaced by new ones.

If you update with synaptic (reload, mark all upgrades, apply) or with apt from terminal (sudo apt-get upgrade), you will be informed about disk space taken or released.

You can also configure your update manager to check for updates only once per two days, or once per week.

Not sure this is true. In my /var/cache/apt/archives there are .debs for multiple versions of many packages, totaling 2.4G now since I started with Gutsy tribe 5. I'm not sure there is anything that limits this by default. Looking into it, I found that the following command will clear out the cache of useless versions:

sudo apt-get autoclean

This reduced my cache down to 1.2G.
I don't know why update manager isn't running this after every update, maybe it is a bug?

I've just turned on a feisty i haven't used for two months, and there was only one version for each of the packages in /var/cache/apt/archives (that system has been updated many times before). So i guess those get deleted after some time, a month maybe.

Anyway this should not be a concern with feisty, as there aren't really that many updates that would make it matter (unless you're very low on disk space), of course with a development release it's a different story.

Apt cache normally takes some space, around 1gb usually, but it can be useful for reinstalling things. nevertheless, if you wish, you can totally clear it from synaptic or by issuing: "sudo apt-get clean".

Some more advanced notes:
You can configure synaptic, configuration-->preferences-->files, to delete packages just after installation. You can enable periodic autoclean in /etc/apt/apt,conf.d/10periodic, there are some settings that seem significant in /etc/apt/apt.conf.d/20archives also (in gutsy), but these appear not to be documented anywhere.