I don't think it will be too long. I have no idea how many episodes it will take to get rid of Aizen, but i don't suppose they will face him now; they'll probably rescue Rukia & flee to the real World or to Soul Society.
Besides, i think that the Hougyoku needs some time to fully awaken, so there's no reason to fight him before their time ends; Ichigo's reiatsu won't even manage to get close to Aizen's right now, but that's a whole different story.
But anyhow, Aizen is planning to defeat "god" himself, by entering his palace with the King's Key. After beating Aizen, who is after the creator, will there be something else to do? Is there anyone even stronger than him about to pop out? It happens in many Anime series; the hero beats the all powerful villain & then an even stronger appears.... how the heck???
What I'm more worried about, when it comes to the end of Bleach is something Orihime said:
Spoiler: I'll use my power to prevent the Hougyoku from having ever been made.
I don't now if she can do it, but thinking about it, there's a possibility that she won't accept to go back with Ichigo & the rest.
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