This is not a functional and neither enabling of understanding question since there is no ability to understand the highness of Satan in a quantitative measure, to state a chronological point or human sense, since this is not told in a direct and specific way as well. Satan is supremely high and not abiding by our own human frame of birth-death cycle.
However as Korpi stated, if it makes sense to you, you can understand this embodiment of human to immortal, nothing to all, as it is a prevalent motive of reality shown to us by the Gods. Not only as a possibility and potential, freedom, but as also creating the intimacy for us humans to understand and be appropriated into the highest dimensions of Satan, that is, Satan has known and has experienced what we all humans overcome now, and from the point "zero" and limited existence, one has broke and conquered all that is. This is an embodiment of the Ether element, and also Shiva shows this directly as a story of reality for our minds.
Now to also entertain a personal curiosity by this question, is to ask personally Satan this, and abide by His sacred and holy nature the answer.
We are so little and small, the only true answer to this that can satisfy reality to the deserving degree, Satan, has existed and will exist for infinity, and we don't understand now the infinity. One cannot understand 500.000 of years as an advancing being of that calibre, but only in a conceptual "infinity".
Also, remember!! All Gods go to Satan for advancement as well. It is not possible to understand such highness of existence. Our point of consciousness is unable to reflect this, since it is Limited and Satan is Unlimited.