Backing up a SketchUp file or restoring an Auto-save file
SketchUp creates a backup file the second time you successfully save your SketchUp file and any subsequent saves. This file is an exact copy of the previously saved version of the file. The backup file uses the naming convention FILENAME.skb on Windows and FILENAME~.skp on OS X, and it's located in the same folder as the original file.
By default, SketchUp automatically saves your files every five minutes while you're actively working. If, for example, you're working on a file named FILENAME.skp, the automatic save creates a file named AutoSave_FILENAME.skp. The auto-save file is kept until you successfully save your original file or specifically confirm that you don't want to save changes. If SketchUp crashes while you're working on a model the auto-save file is not deleted. You can recover your work from the point the last auto-save was performed by opening the auto-save file in the same location as your original file. If you've never saved your work the auto-save file is stored in your "My Documents" folder.