There is a thread about a level 17 rogue with 122 hps yet I was in a quest with a level 18 5 time tr cleric that had 128hps. He said he does not need fort or hps as he stays out of the melee and is an excellent healer. I mentioned the fact that it hard to heal if you dead.
The truth is people need to understand that you should start with 14 con on any character (even a drow). You need to take toughness (except maybe on a character with really high hps like a barb where it may not be necessary but even then I say take it). You should get a +6 con item and a greater fase life item as soon as possible and by level 15 there are many availavle on AH. Of course there is more things then that like minos but then to get minos and gs hp item you need packs so it not mandatory. But +6 con, greater false life are necesssary.
So lets look at numbers.
starting con 14 +6 con item gives 20 con. So at level 18 you would get 90 from that. Greater false life gives 30 more. 20 more from toughness feat 3+17. wizard gets the lowest hitpoints so d4 per level so 72. THen action points on race toughness 20hps. So even a level 18 wizard should have at lest 232hps and all characters should have that much at least.
Add in toughness on a minos 20, 45 grom a gs, 20 more on a guild slot item and 317 is fairly easy to achieve. But at 18 you have no excuse to have at least 200 and heavy fort and should have more no matter what class.