Aimei & Shanna> This is wat i gather from Beauty Aurora (BA) and my own experience in facial salons.
BA told me that she uses an ampoule once every 2 days to hydrate her skin. Ampoules are concentrations of highly active ingredients to address every skin condition and deficiency. There are various types of ampoules in the market so you'll have to find one that suits ur skin condition.
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Personally, i find that I cant finish one ampoule in one use so I split that up to 1/2 ampoule per day. It last me longer in my supplies [saves costs =)] I'm using the sensitive ampoule as I have sensitive skin. So far, I did not develop any adverse reaction to the ampoule I have bought online.
If you visited any facial salon, they will also use ampoules during the session as well. It ultimately depends on your budget (one ampoule may cost abt $1 to $3.50) as well as how much you need to work on your skin condition if you buy online.
However, it's always best to start early. As the saying goes, "Prevention is better than cure". =D