Can anyone tell me if i want to translate the complete org into 10 different languages how much time will it take?
It may take a few hours or a few weeks (or even longer). There's no way we could tell you how long it would take without knowing more about your org. Your best bet would be to download the translation files to get an estimate.
Is there any elemnts in org that cannot be translated?
As far as the front end user interface, you can translate most elements. I don't know if there's something you have in mind that's not translatable, but the list of things you can translate should pretty much cover anything you'd want to translate.
If i am translating the org for the first time and using translation workbench then will it take more time?
It should take approximately the same amount of time overall, though the stf files will probably be faster in the long run if you have many elements.
How to edit the stf files ?
Preferably, use a translation software package designed for the purpose of editing stf files. Note that Microsoft Excel can cause errors in the file, and Microsoft Word is also likely to cause problems in the file; you may be required to open the file in a text editor to fix these problems. You might try OmegaT or another translation software suite (salesforce.com mentions SDL Trados).