As early as you like if you fancy ALOT of running and dying. Basicly you gain yourself access to the Necrohol of Nabudis and make a run for it!

I just found out you can actually get TWO Zodiac Spears. There is another in the Henne Mines but you must have the Diamond Armlet equipted and even then its still like 10% of getting it.

Ok so where do i find the Necrohol of Nabudis? Can i get it just after the Raithwall tomb?

Yeah i read that, its actually a 1% chance you have of getting it, and you have to have gained 10 espers and something else..

There is a chance that once you've acquired access to the Yoma in Estersand via completion of the Cactoid & Earth Tyrant sidequests, you can enter such areas as Mosphoran Highwaste and Salikawood very early in the game. But a lvl 30+ boss is guarding a path that leads to the Necrohol from where the weapon lies. And even if you defeat it, you still have to make your way past enemies who are disturbingly strong for their levels unless you've learnt a certain spell.

I managed to get it with 3 characters at level 39 and the others in the 10s following the "storyline route" not the "backdoor route" like I suggested above. But seeing how simple it was to flee with minimal damage incurred, doing it at much lower levels might also be realistic as well.

Note: Should any one of the specific treasure containers which prevents the Zodiac Spear from appearing in Necrohol be opened, you can kiss that one goodbye.