Somewhere between 500 and 1000 depending on the optimization of the party.
The NG's problem is that by forcing that obnoxious Strength check (which requires something like 700 str), it throws its CR way out of line, as none of its other abilities require even close to that kind of power to overcome. Not to mention, a being who could actually make that check would be able to overkill the NG a thousand times over by flicking it thanks to VSCs.
None of its other abilities really even come into play against an Eternal enemy. DR 1500/-- is worthless. Fast Healing 1500 out of... 1.5 millions HP. Right. The DCs for Gravitic Aura aren't out of line for a powerful Sidereal to make reliably. 62d1000 Divine Fire damage per round is...okay I guess, for subduing Sidereals, but 620d1000 might actually make an Eternal notice the thing (which at that CR ought to be the target). Immunity to magic is blah, wasn't even much of a problem at low levels. Pulsed X-ray is...really it's only decent form of attack. And its Death Throes...even 200 damage isn't enough to cripple an Eternal's ability scores.
Anyway, if a party has things like Abrogate and 7th/8th/9th sense (which they should at this point!) the thing is kind of a joke.
Holistically, it probably SHOULD be around CR 900 or so, powerful enough to give even parties of Demiurge pause. But to do that, in my opinion, it would need a lot of work. (I'd give it 1,000 HD or so, infinite Fast Healing, DR 1/10th or so, and maybe fiddle with its offenses if the HD boost didn't buff them enough.)
But that's just my take on it. Pretty sure Paradox has had his party actually fight them.