Save your work regularly so that you do not have to redo it in case of a system crash. This is why when working on a projects, writing long e-mails or documents, you should save frequently.
Recommended save duration
Simply every page you type, or every 10 minutes, which ever comes first for you.
Quick tip: The shortcut as you go
To save: Press Control + S
File > Save
If you are saving the document for the first time it will ask you where you wish to save it as. If you are using either of the above procedures you will not see any kind of confirmation that it has been saved – but it will have.