If I remember rightly you get one free update of data and obstacles with a code which comes with the unit when you buy it new. The update is downloadable from the Garmin website free for the first time, and chargeable thereafter.
I think the terrain data is permanent although would probably be reviewed from time to time to allow for Icelandic volcanoes and the like
.
For what it is worth when I first bought my unit with the old software installed (presumably the data goes out of date because the units are inevitably stored for some time between manufacture and purchase) the unit kept crashing and switching itself off. Others have made the same complaint. However, it has not done so since I downloaded the latest version 2.5 software.