OS X Mavericks: About local snapshots
If you have a portable Mac, Time Machine makes copies, each hour, of files that have changed. These “local snapshots” are stored on your computer’s internal drive, and are separate from backups stored on your backup disk. (Local snapshots are not saved on desktop Macs.)
A single daily snapshot is saved on your computer’s internal drive every 24 hours, counting from the time you start or restart your computer. A single weekly snapshot is saved for one week.
If the space on your computer’s internal drive becomes very low, Time Machine performs additional thinning:
When the disk’s free space is less than 20% of its total available space, Time Machine removes snapshots starting with the oldest one and working toward the newest one. Once free space is more than 20%, Time Machine stops removing snapshots.
If the disk’s free space falls below 10% or is less than 5 GB, removing snapshots becomes a high priority for Time Machine. When free space is between 10%–20%, Time Machine continues removing snapshots, but assigns this task a lower priority.
If Time Machine can’t free up enough space to get above the 10% or 5 GB threshold, Time Machine removes all snapshots except the most recent one, and stops creating new snapshots. Once free space rises above the 10% or 5 GB threshold, a new snapshot is created, and the previous one is removed.
To stop saving local snapshots, open Time Machine preferences and slide the switch to Off. Snapshots resume when you turn Time Machine back on.