alleria was with the army of the light and turalyon for 500 years
Alleria and Turalyon had lived through more than five hundred years in the Twisting Nether. For Azeroth, it had been about twenty since the end of the Second War, barely a single generation. Its champions were facing another critical war.
then she spent 500 years on her own
Being captured had not exactly been part of her plan, but circumstances had required some improvisation. If nothing else, it had brought her close to her prize. Five hundred years of searching had passed. Now her goal was within reach.
and after that (which is all assumptions from now on because there’s no defined timeline)
she spend a couple months in niskara. I’m assuming it wasn’t just days/weeks because she says the demons already grow frustrated with her not telling them anything ("Good." Alleria had kept watch on the Legion's interrogators. Over the past few days, they had grown visibly frustrated that her will had not been broken. Time was short.)
she goes with the locus-walker to the void, where he trains her. I have no idea how time passes in the void (is time even something that applies there?), but considering it was time enough for her to be trained and do everything to master it except the very last part of her giving herself to the void which she does later with l’ura and etc, I’d say it was the equivalent to a couple centuries at least. Maybe around 300-400?
she’s warned her training is going too fast, and ends up getting the vision of the demon ripping off turalyon’s soul, and goes back to the xenedar to save him
xe’ra locks her up. it isn’t clear for how long, as the first mention we have of it is dialogue between lothraxion and turalyon that merely states she had still been imprisoned when the xenedar crashed. when alleria speaks of it, there’s no indication of time either (Alleria Windrunner says: And what then? I was locked in a cell for delving into the Void against her wishes. Will she make me a prisoner again? Or worse?). For my interpretation, I choose to have it as being at least a couple centuries too, though less than the time she spent with the locus-walker in the void. Between 100-200 years, perhaps.
it took her two years to wield the light as a weapon so well that she no longer needed her bow and used it just because she wanted to (Within two years—from Alleria and Turalyon's perspective—her arrows were wreathed in the power of the Light. She could have forgone her bow and arrows altogether, but it pleased her to bring Thas'dorah, the legacy of her family, into battle against the forces of evil.)
when the sunwell is destroyed, she’s with the army of the light still.
by the time of tbc, she and turalyon go to argus for the first time. it is then that they get separated, and she leaves to look for the locus-walker. she leaves before the sunwell is restored.
