These #s can vary wildly depending on usage. I’m advising most of our users to ensure their machine have minimum free space of 2GB, and recommending 10GB to accommodate updates, library contents, and backups.
Normally, iTunes only keeps the most recent backup. On MAC OS X, users can let TimeMachine collect multiple backups for them. On WinOS users can manually copy the backup folder which is created under their “my docs” profile folder.
My personal iPad syncs with a MAC; even though that iPad has about 30GB of content, the backup is only 303.6MB
Content currently loaded includes 74 apps, 1358 songs, 5 movies, 2 tv shows, 416 books, and 4074 photos.
The work issued iPad currently has about 6.7GB loaded. Contents include 44 apps, no music movies or tv, about 1100 books, 40 photos. This iPad syncs with iTunes on Windows and a backup is ~5GB.
Why the difference? Why does backing up to iTunes on a Windows machine require so much more disk space?
Mac OS X is very efficient at utilizing symbolic links (these are sort of like a shortcut reference to a file or object in another location). When running on a MAC, iTunes can build a list of symbolic references for the objects in your iPad (or iPhone). This enables a backup to contain a list of the original files at their real location combined with only the unique iPad data not already on your desktop computer.
On Windows, the operating system doesn’t provide iTunes a mechanism to make these symbolic link references. So iTunes needs to backup everything on the device. It will try to compress what it can, but most media content is already in a compressed format.
If you’re setting up a new iPad and iTunes installation for someone who is likely to sync a lot of multi-media content to their 64GB iPad, should probably plan to see 100GB to 200GB of disk space consumed by the iTunes library and the iPad backups.
Fortunately most new laptops come with a hard drive capacity of 300GB to 500GB. Folks who maintain larger media library (lots of movies, tv shows, video podcasts, etc.,) will either need to look at larger external disk drives (1.5TB to 2TB) or maybe even a storage array. Considering the cost of obtaining content, it’s also a good idea to plan for a second drive or array for backups. MAC OS X Time Machine works quite well when using a single large external drive as a backup target, but folks with multi-terrabyte media collections will need to investigate more complicated storage and backup solutions.