Are you asking about OS updates, or application updates?
If applications, you have two options; in the Play Store app, you can choose automatic updates or manual updates. If you choose automatic, whenever there's an app update it will just happen. If you choose manual, you'll be notified when updates are available, and you can choose whether to update each app individually.
If you're talking OS updates, things are a little bit more complicated. If you don't root your phone (more or less like jailbreaking an iPhone), you'll get OTAs, as they become available, with no manual intervention. If your phone is rooted, OTA's will typically fail. But manual updates are usually available very quickly, often before the OTA has rolled out to everyone. If you've installed a custom ROM, then you'll need to wait for your ROM developer to incorporate the update and release a new version.
One other thing about OS updates: Unless you buy the unbranded international version of the phone (i9300), you're always dependent on your carrier to release an OS update. You won't get it directly from Samsung.