As stated, this is going to depend on the era/tech/faction, as well as the Fighter Size.
Sure, my 3068 Heavy Ferro Super-Stuka might be able to slap 141 points of armor on the nose to be immune to 15 pt hits, but that is going to be rare.
Most of my Heavy Custom Fighter Designs, I shoot for 91 Nose, 71 Wings, 41 Rear as good goal posts. More is fine, but this is a solid amount for an "assault" bird.
For Mediums, I'm happy if I can pick up 71+41+41
Lights, 41+41+21
Taking Clans into account the big difference is the 61 needed for MLs doing 7 point hits. But I can normally get that through Clan Ferro easy enough.