[Boromir88]

I don't think it's ever really conclusive when Saruman had started heading down the wrong path, but I think we can get a generalized idea of the time frame.

We know that right from the start Saruman (Curumo) was jealous of Gandalf (Olorin):

But at that Varda looked up and said: "Not as the third;" and Curumo remembered it.~Unfinished Tales; The Istari

And when Gandalf recieved one of the Elven Rings, Saruman found out and grew even more jealous:

'And the Grey Messenger took the ring , and kept it secret; yet the White Messenger (who was skilled to uncover all secrets) after a time became aware of this gift, and begrudged it, and it was the beginning of the hidden ill-will that he bore the Gret,which afterwards became manifest.'~ibid

I don't think Saruman was evil at this point, but I think we can definitely see the beginning of his turn to evil right from the beginning with his jealousy towards Gandalf and the Ring he recieved from Cirdan.

'In this way Saruman began to behave as a lord of Men; for at first he held Isengard as a lieutenant of the Steward and warden of the tower. But Frealaf was as glad as Beren to have this so, and to know that Isengard was in the hands of a strong friend. A friend he long seemed, and maybe in the beginning he was in truth.'~Appendix A: The House of Eorl

During Frealaf's reign when Saruman is given Orthanc (T.A. 2759), Saruman still appears to be a good friend to everyone. He at first wtaches over it as a 'lieutenant' to the Steward Beren...then it appears Saruman starts having a change of plans. As Raynor shows in the T.A. 2851, Saruman had started thinking about using the Ring for himself.

So, roughly during that 100 year period when he first is given Orthanc, to the meeting with the White Council I think we can say is when Saruman officially became corrupted. However, I think the seeds of corruption were sewn much earlier starting with Saruman's jealousy of Gandalf.