I don't know of a built in function so I would use VBA to work out how many characters the document has and iterate through the tracked revisions to add up the characters that have changed.
I'm not how to interpret the result though.
For instance if half of the document has deleted text and replaced it with the same volume of text, does this mean 100% change? If we deleted half a document then the maths makes sense but it isn't as easy to arrive at a meaningful result if there are deletions and additions unless you report on those separately.
Code:
Sub WhoMovedMyCheese()
Dim lngAdd As Long, lngDel As Long, lngOther As Long
Dim aRev As Revision, lngDoc As Long
For Each aRev In ActiveDocument.Revisions
If aRev.Type = wdRevisionDelete Then
lngDel = lngDel + aRev.Range.Characters.Count
ElseIf aRev.Type = wdRevisionInsert Then
lngAdd = lngAdd + aRev.Range.Characters.Count
Else
lngOther = aRev.Range.Characters.Count
End If
Next aRev
lngDoc = ActiveDocument.Characters.Count
MsgBox "The document character count is: " & lngDoc & vbCr & _
"Additions: " & lngAdd & vbCr & _
"Deletions: " & lngDel & vbCr & _
"Other changes: " & lngOther, vbInformation + vbOKOnly, "Changes"
End Sub