It took me 4 elixirs to +4, then 1 astral to +5 which was saved. Then I took lucky stone and got it +6 and it didn't lose the astral. Then after a month or so, we went for +7 with another lucky stone added and it went +7
From what I have noticed though, the more astrals you put, the higher the chances of alchemy failing. This is what I just noticed from my past experience. So I try 1 astral / 1 immortal each time instead of putting all of them at once.