My first two fiction books are with a top publishing house. They have had them for three months. Six weeks ago, they told me they were prioritising looking at them. How long on average does a major house 'consider' a book before rejecting or accepting it? As I haven't heard anything since, does this mean after all this time they don't want them, or could they still be considering them both?
I'm not sure how the knowledge of an "average consideration time" could help you even if it were feasible to figure one out. As said in the comments: Maybe they never answer. And why should you care when they do?
There is no need waiting for them. Put your work in front of as many people who are able to buy it or self-publish it. Or both. Honestly self-publishing has a big advantage: if your books sell, you have a much better negotiation start point. You can prove that your novel sells, what makes it much more attractive for a publisher.
If they don't answer you can ask them what's up. Maybe they think it is impolite, because you disturb them in their working process. But letting you wait is impolite also. And it's still possible that they never answer because they gave up sending out tons of rejection letters and do not know how to use an email template for that. Or they just don't care.
Go, try to sell your books, one way or the other and don't care about them. Which means: send it out to other publishers.