Update from ios 6 to ios 8

I have an iPad 4. Still have ios 6. Want apps I can't purchase since they require ios 7. How much space am I losing to upgrade to current ios 8 version?

You can see below the free space available with iOS 8 for each kind of devices :

Edit :

Table from http://www.techrepublic.com/article/lawsuit-against-apple-for-ios-storage-misrepresentation-get-the-facts/

Hi i also have an ipad running ios 6 and i want to update it to Ios 8.4 It's still updating right now through itunes on my laptop and not through air. It says here that it will take 1.67 GB and it takes 2-5hours of download, i hope this helped