OS X Mountain Lion: About local snapshots
If you have a portable computer, Time Machine keeps a copy of everything on your computer’s internal disk, and saves hourly “snapshots” of files that have changed. Because Time Machine stores the snapshots on your computer’s internal drive, they are called “local snapshots” to distinguish them from backups stored on an external backup drive. Time Machine saves local snapshots on portable computers only, and not on desktop computers.
Local snapshots are periodically thinned to save disk space. A single daily snapshot is saved for every 24 hours, counting from the time you start or restart your computer. Similarly, a single weekly snapshot is saved for one week.
If disk space becomes very low, additional thinning is done:
When the disk’s free space is less than 20% of its total available space, Time Machine removes snapshots starting with the oldest one first, and working toward the newest one. If enough space becomes free so that the disk’s free space is more than 20% of its total available space, Time Machine stops removing snapshots.
If the disk’s free space falls below 10% of its total available space, or is less than 5 GB, the task of removing snapshots will be given high priority on your Mac. When free space is between 10%–20% of total available space, removal continues at a lower priority.
If Time Machine is unable to free up enough space to get above the 10% or 5 GB threshold, Time Machine removes all snapshots except the current one, and stops creating new snapshots. Once free space rises above the 10% / 5 GB threshold, a new snapshot is created, and the previous one is removed.
If you want Time Machine to stop saving local snapshots, open Time Machine preferences and slide the switch to Off. Snapshots will resume when you turn Time Machine back on.