How high a % of glicolic acid cream is safe to use just at home?

Glycolic Acid. This answer is not a simple one. Many of the over the 'acids' are partially neutralized so the percentage that they are claiming on the package is not the actual percentage. Another factor to consider is any underlying skin conditions such as rosacea or excema that can be flared up by the active ingredients. I would recommend doing a 'patch test' on a small area of skin of the neck.