You need to consider several things.
The rules for attribute drains presume temporary loss to attributes. This goes back to the d20 SRD, and is inherited by Pathfinder. In Pathfinder, according to the Beta, it's restored at 1 point per day of bedrest.
the penalty for attribute drains is not the same as the penalty for a low attribute, even tho' it is related conceptually and is parallel in magnitude. For a character with an even attribute, this actually is less severe by one point than for a refiguring of attribute modifier and its application; its the same for characters whose base value is odd.
The reduction in Max and Current HP can be seen as a systemic shock, rather than direct damage, but it automatically is restored as the con damage is healed.
If dead, you stop healing. But death is not until HP = -10 or -(Con), whichever is further from zero.
1st level HP are maxed, and the worst is a d6.
Remembering that Con drain is temporary, healed by bedrest...
The worst case is a Con 11 wizard (+0 HP/HD from Con), and has minimum HP every level, who takes 10 point loss to Con (-5 HP/HD from drain).
At level 1, HP=6 he's merely reduced to 1 Max HP.
At level 2, with minimum 11 HP, he's merely reduced to 2 Max HP.
At level 3, with minimum 12 HP he's merely reduced to -3 Max HP. Coma - see below. Enters coma when Con drain is 8 points.
At level 4, with the minimum 13 HP, he's reduced to -8 Max HP. Coma - See Below. Enters coma when Con drain is 6 points. But note - he could have raised con, and have Con 12, and been minimum 17 HP, and thus not be subject to coma until Con drain hits 8 points.
At level 5, with the minimum 14 HP, he's reduced to -11 Max HP. Dead. Enters coma when Con drain is 6 points, for max HP=-1.
At level 6, with the minimum 15 HP, he's reduced to -15 Max HP. Dead. Enters coma when Con drain is 6 points, for max HP=-3.
At level 7, with the minimum 16 HP, he's reduced to -19 Max HP. Dead. Enters coma when Con drain is 6 points, for max HP=-2.
At level 8, with the minimum 17 HP, he's reduced to -23 Max HP. Dead. Enters coma when Con drain is 6 points, for max HP=-5.
At level 9, with the minimum 18 HP, he's reduced to -27 Max HP. Dead. Enters coma when Con drain is 4 points, for max HP=0.
Coma - the rules don't call it this, and it's an interesting effect that occurs. If your con is drained, and your Max HP drop below 0, you aren't dead if someone stablizes you before the death point is reached. Sure, you can't heal until the con comes back. You're unconscious, immobile, and will need special care. But if tended, you WILL get better, when your con returns.
Note that some things that can keep you from entering a coma state - Temporary hit points being the most obvious - and temporary boosts to Con. Or any ability that allows you to act while HP is below 0.