Well, let's do some quick (and simplified) math!

Since the game has gone on so long, let's assume it's a pre-30 game; they just don't know how important tower pushing is yet. So let's also assume they have no runes and masteries.

Nasus's Q does 226 + X, where X is 3 times your number of stacks (plus extra for big minions)

Sion's health is 2275 + Y, where Y is 3 times your number of stacks (a bit less because it starts lower, but we'll assume they both level to 18 before farming, and because as the number of stacks approaches infinity, the average health/kill will approach 3) and his armor is 176.25. So Sion's effective health vs physical damage is (2275+X)*2.7625

But the number of stacks can be calculated as a function of time. Let's assume that Nasus has full CDR for stacking his Q, but no damage CDR; Frozen Heart, Frozen Fist, and Shurelia's (none of these affect the damage calculations we're going to do; Spirit Visage would do so greatly).

So Sion will get 13 stacks per minute (assuming we're past the 25 minute mark, which is, again, pretty fair, given that our stacking is infinite), for 39 health per minute.

Nasus, on the other hand, gets 42 bonus Q damage per minute!

Don't be fooled into thinking that this means that Nasus can ever catch up to Sion. With all of that armor, the effective health gains of 39 health are about 108.

On top of this, because there are no cooldowns for Sion, he could cheese these numbers by visiting other lanes where big waves have built up, and returning. Due to CDR constraints, Nasus is capped at 25 stacks per minute, and even if ALL of those are cannon creeps (perhaps he has his allies freeze lanes forever, but kill all of the ranged/melee minions, until there is nothing but millions of cannon creeps waiting to be Q last-hit?), the best he can get is 150 damage on his Q per minute, which still barely beats Sion's simple effective health gains of 108; if Sion pulls similar shenanigans (while having his allies keep the WHOLE wave alive), he can approach 324 health per minute.

So if the question is 'how long do Nasus and Sion have to charge their stacks until Nasus can one-shot Sion?', the answer is 'never'. How much would Nasus have to charge his Q stacks to kill himself? Well, with a base health of 2030, and a base lv 18 MR of 52, he would need to do 3085 damage to kill himself. Since Thornmail reflects 30% of damage back at him, he needs to do 3085*10/3, or 10285 damage. Damn. Assuming he is gaining 150 damage per minute (the stacked cannon minions plan), he will be ready to one-shot himself after 69 minutes of perfect Q-farming (although the animation will add a tad of time).

In only one lane, with only 42 stacks per minute, this will take him about 245 minutes!

Just kidding. You just did 10285 damage? Divide that by 2.7625 for all that armor, and then multiply it by 20% for lifesteal, and you healed 745 damage. Phew, you just barely lived! Go charge up a few more Q stacks

But let's assume that we're not asking about charging time. We just want to know, academically, as the limit of both stacks approach infinity, what happens?

Nasus strikes Sion, doing damage equal to *as much health as Sion has, whatever it is, times 2.7625*. Sion dies INSTANTLY. 3/10 of that damage returns to Nasus. For every 100 health Sion just took, Nasus takes about 60, after 52 MR. In addition, he lifesteals 20% of the damage dealt POST-armor, or about 20 for every hundred damage Sion just took. Nasus takes about 40 damage for every 100 damage he has just dealt. Unfortunately, 40% of infinity is a lot of damage, so Nasus immediately dies.

So what's the answer? Sion dies for sure. That's what infinite damage has to mean. He takes 2.7625 times his health in damage, and keels over.

Nasus lifesteals for 20% of Sion's health, but takes 82.875% of his health in damage. He'd need 83% lifesteal to survive (and once he had that, he'd never leave full health.

Oh, and I'm pretty sure it's irrelevant. Why? Because either the lifesteal happens first (wasting it), or it happens after thornmail (in which case you're dead before it can happen)