How much should I spend on renovation?

Someone once told me a guideline that renovation should be no more than 10% of the purchase of the house. This is to ensure you still have a comfortable margin for profit if / when you sell the place.

Note: this is just for the renovation i.e. flooring, built in wardrobe, electrical etc. i.e. the stuff you cannot bring with you if / when you move. This does not include furniture, appliances that you can bring to your new place if you move (i.e. your bed, sofa, etc.)