A month's trekking in Australia is perfectly good English. A month trekking in Australia is elliptical (or short) for A month spent trekking in Australia, and is OK as well, as is also a month of trekking in Oz.
"Two hours' English" sounds odd because English is not a gerund like trekking. If it is understandable through the context, I might use it. Though I find "two hours of English" preferable.
"After a few hours' wait" also sounds acceptable to me.