Okay so, lets look at the ability, it gives 2 health for killing a small minion, but 8 for killing a large minion/monster or getting a kill or assist. So if you use the jungle to calculate:
In the jungle on a 1 minute cooldown there are 4 large creatures, and 6 small creatures, so that gets him 44 health per jungle clear.
4 x 8 = 32
6 x 2 = 12
32 + 12 = 44
BigWraith + Wolves + Wraiths + Golems = 10cs
If he is jungling he will get blue and red each 5 minutes, so as a jungle he gets 244 health in 5 minutes (with perfect clear) from 56 cs. The aim of getting 595 health on his soul furnace should therefore only take
44 x 5 + 2 x 8 + 2 x 4 = 220 + 16 + 8 = 244 health
10 x 5 + 6 = 56 cs
This means with perfect clear it will take in 12.2 minutes (132 cs), assuming no kills, and no lane tax (both unlikely). Now you might say, "he would get counter-jungled" or "His clear is slower than that" so lets handle that, I would put in a factor of 25% loss, so that's an extra 3 minutes or so, to make it 15.24 minutes (However, this won't increase his cs since he won't be getting the health from those lost monsters). Most games take longer than 15 minutes, 20-30 minutes, which means in that time (give lane tax and kills/assists) he will easily surpass his old base.
"But wait" you say, "old sion also scaled his health from kills, you have you calculate that to.". Alright, I'll do that, old Sion, if he starts the game with his Enrage maxed then he gets 3 health per minion kill.
This means that with the 132 cs he would have 396 health, way less than the 595 of the new Sion, "but how long until new Sion's max health is more than old Sion's"?
Alright overly curious imaginary questioner, lets figure that out to. For each set of 5 minutes in the jungle new Sion gets 244, while old Sion got 168 (assuming his enrage was maxed from level 1, instead of level 13 or later).
However, if we are going to do this, we should take into account the lower early health scaling from old Sion, to do this I have used a random ap sion build guide (since Commando Yi was talking about AP Yi), this guide doesn't level his enrage until level 4 (so he doesn't get any health from his first clear) and maxes it at level 18, so I am going to say at 25 minutes the sion will be level 18 (way earlier than is true almost always).
Calculating this gives it that after 15 minutes new Sion in-fact has 7 more health than old Sion did, of course this is after the jungle first spawns.
Edit: Alright, I just had a look at an ad Sion build which maxes Enrage first, assuming you do that we will treat it as a transition from 1 to 3 per cs at 15 minutes, (So I am giving him the full 15 minute clear at 3 health per cs). This gives old Sion more health until 20 minutes in, so this gets old sion with more health for 5 extra minutes, I don't think that's enough to win a game.