I've done a few untwinked p8 HC characters, but not single pass though. Hell mode was most difficult, as expected, with the high life pool of monsters. Caster builds will have it easier though. For my Zon, CS/LF melted norm and nm even on p8. Had to use FA for the lightning immunes in hell, and that was where things slowed down.
On the melee side, my barb struggled with doing enough damage in the later stages of hell, but crushing blow was a big help there. Farming for decent weapons will take some patience (or luck), but not an option in single pass play. A source of poison or OW was critical in stopping life regeneration. Kept a Woestave in stash for boss fights - maybe the slow was cheesy but it was safe and kept my merc alive.
I don't remember how long it took, but a single Act could take a couple of hours if you're killing most monsters.
I used to play a lot of p8 single-pass full-clear untwinked characters, usually with tournament rescrictions. I couldn't tell you how long it generally took except in a very generic "very very long time" sort of way. These days I still play a fair bit of untwinked, but I no longer worry about single-pass full-clear as I found it encouraged behaviour/play-styles in me that ended up being very tedious.
Most characters with p8 single-pass full-clear untwinked with inevitably hit a wall. Sometimes that's late NM, sometimes it's act1 hell, sometimes it's act 3 hell, but when it happens everything slows way way down. Sometimes, rules permitting, that means it's time to drop to p1/p3, or a change in strategy, but sometimes it just means buckle down and push through because there's no way to make progress not tedious. The shining example of this to me is a naked druid character I have from a tournament years and years ago. He's still alive in late NM. The game still feels very possible/doable, and not particularly 'hard' yet, but things have slowed down so much it's more a matter of how much tedium I'm willing to endure to get to that next waypoint or quest completed. So much of the game at that point is just sit and wait for things to slowly die, but being a HC naked challenge you can't afford to have your attention lapse too much. Paying such close attention for threats makes one very aware of how very slowly everything is dying, which can be frustrating. It can, with the right frame of mind, be rather relaxing or enjoyable, but there's a reason folks like InsaneWayne got/gets so much props. (A good read if you're looking for some AARs of some p7 untwinked single pass no-gear play).
In broad generalities, NM usually goes the fastest, as most characters will have the core of their build well on the way and a decent set of options for gear. Normal depends a little bit on luck in terms of a lucky early drop can make a big difference, but more so on at what level you start to hit abilities you want to be putting points into, and the quality of abilities along the way to them. This can vary greatly even just between trees in the same class. Hell has a pretty good chance of being rough for any character between immunities, struggling to get enough resistances, and the monsters being beefed up in general.
That said though, the slower pace of the game, and generally more challenging experience is a very different and equally enjoyable version of diablo 2. And what in one mood is tedium in the etxreme, in another mood is very enjoyable.
P8 untwinked all the way can be done. The real question is: why would you want to? A skelliemancer for example would have great difficulty with the maggot lair. In the end you can do it, but you'd probably want to stab yourself in the eye before finishing.
So all in all, with the of some tournaments, i'd strongly advise against doing so.
It depends on whether you want to do full clears or not. With full clears, the average time to finish all Acts in all difficulties untwinked at players 8 in Normal and NM and players 1 or 3 in Hell was like 100 to 120 hours IIRC. It will be much faster though without full clears. Take the false tombs in Act II for example : you can skip those, whereas with full clears in Hell I need like 4 hours for all tombs, depending on the build. Of course times might be faster without tournament restrictions - my times were for tournament characters. Someone should ask jiansonz - he will have more detailed information about this topic
EDIT: And no, I never did players 8 untwinked single pass through all of Hell. I think once I did the first 2 Acts that way, but got bored because it took ages Then I switched to /P3
EDIT2: I had a look: It was more like 130 hours+ I needed for single pass, untwinked, HC players 8 (Normal, NM) players 3 (Hell) full clears. Man, did I have time to spend back then...
I was untwinked until ATMA came along and helped things...like sharing items and stash management. Finally my sorc could get my necro that awesome wand she found!
Nowadays I have enough RL restrictions on how much I can play I choose to use my time on the tasks that are more important to me as it relates to this game, like getting a character to 99, finishing my grail, experiencing other builds besides the main ones I play.
Before Atma I had 2 pcs, but 1 monitor. I'd fire up one, physically plug the monitor into the 2nd, join game, drop items .. move plug etc. I loved being able to get all my gems from various characters and cube them to their potential! To be able to complete basic sets, or get closer to it .. or just to be able to have extra space
These days I have too many characters, not enough time. I have over 100 characters started but not finished. From act 1 normal up to act 5 hell. If I play just one in a week thats 2 years to cycle through all of them.
My goal is to one day finish all 100 in the same long weekend! Imagine that for a Pat/Mat thread!