How Long Does Incubation Take?
There is no "hard" answer to that, and no rules. We've had a project go through incubation in 2 weeks, one do it in 6 months, another in 7 months. Others take a year or more. The incubator has some entry criteria and some exit criteria (see the policy docs). Any project that satisfies the entry criteria can enter, any project that satisfies the exit criteria can (will) exit.
What lies in between? If all goes well, we see some (difficult-to-measure) "progress" over time towards a project meeting the exit criteria. If progress halts or unresolvable community problem arises, the project can also be retired from incubation. That does not exclude that the project move elsewhere and potentially enter Apache incubation again later. While the time a project spends in the incubator is not indefinite, as long as there is progress, there's no artificial deadline.
This info was distilled from a mailing list conversation on the general mailing list for the incubator.