A general "rule of thumb" is, to keep a reasonable "depth" of backups, Time Machine needs 2 to 4 times as much space as the data it's backing-up (not necessarily the entire size of your internal HD).  Be sure to add the size of the data on any other drives/partitions you want to back up.

But this varies greatly, depending on how you use your Mac.  If you frequently add/update lots of large files, then even 5 times may not be enough.  If you're a light user, you might be able to get 1.5 times to work, but that's subject to problems any time a large backup is needed.

And, of course, the larger the drive, the more old backups Time Machine can keep for you.  A drive that's too small may only have room for a few weeks (or even days) of backups.

Unfortunately, it's rather hard to predict, and most of us have a tendency to add more and more data to our systems over time, so if in doubt, get a bigger one than you think you need now.

Also, there are some OSX features and 3rd-party applications that take up large amounts of backup space, for various reasons.  See question #9  for details.

This is a trade-off between space and how long Time Machine can keep its backups, since it will, by design, eventually use all the space available.  But it won't just quit backing-up when it runs out:  it starts deleting the oldest backups so it can keep making new ones.  Thus, the more space it has, the longer it can keep your backups.

5/2/13

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