Susan is absolutely correct, I own magnesium ribbon, and lighting it with a normal lighter (ie. bic or zippo) it takes about 30 seconds to get hot enough to ignite. When I use a blow torch it ignites within about five to ten seconds.

I seriously doubt a bar of magnesium could ignite in any situation other than direct contact with fire for about a minute.

Also, do not look directly into burning magnesium, it burns very brightly and could damage your eyesight.

Frank