1x diameter on steel will work if everything is perfect. Most of the times not everything is perfect so the rule of thumb is 1.5x diameter. With aluminum you want a little more thread engagement so 1.5 to 2.0 should work. If you're working with plastic or wood then you want even more thread engagement. Really high strength steel with carefully machined threads you could get by with less than 1x diameter but it could be marginal. The engineering answer for any question is "it depends".