The way I interpreted it was that it's been 100 years since Oz was dragged into the Abyss. I think this for several reasons.
1. They refer to everyone following a 100 year cycle constantly, so if Alice's Oz's souls entered the golden Abyss they could be reborn after 100 years and that would fit into the story better than any other explanation for their return.
2. Gil mentions that he watched everyone he loved slowly disappear, indicating that he's been waiting so long he's outlived everyone else.
3. Baskerville's have a very long life span, yet Vincent reached the end of his at the end.
4. Lacie's grave site has changed dramatically. More than what would happen in a few year. It was underground at that and on a hill by the end.
I juz finished it but I dun understand few things..yeah I got the part as Oz got dragged to Abbys at the end but how he able to come back w Alice? Also how many years is it after they restore the world? Gosh, the timeline is confusing me a lot. Especially the ending..