What percentage of anothers painting may be copied and not infringe on their copyright?

Reproducing another artists' painting is a copyright infringement, so is reproducing a published photograph such as one in a magazine. Copyright law says you must change it at least ten per cent to make it your own.

I don't know of any Country where copying a painting is illegal, but copying the artists signature and attempting to display or sell a copy as original is definitely an infringement of copyright.

My advice is to use your own signature and add a note declaring it to be a copy and crediting the original artist.