As to how long these ant mounds take to develop:
The prairie restoration in our front yard/pasture (kept wet by our drain field) has some nice mounds that have developed over the course of 10 to 12 years. They're still active -- whenever I accidentally scalp one with our mower, the ants are visibly annoyed (OK, maybe a bit of anthropomorphizing there) and set about patching the mound.
One ecologist who was working for the Institute for Applied Ecology commented that, in managed prairie restoration, it seems like these mounds develop about as fast as their mowing cycle, on sites where annual burning isn't possible. So these might be an important element of prairie "microtopography" that might be hard to replicate in some prairie restorations.
Regarding places in the Willamette Valley that haven't ever been plowed: The main prairie at Finley is almost certainly the largest remaining example of native wet prairie. There are likely other places that were grazed but not plowed, for example, the upper part of Baskett Butte. The top of Coffin Butte is likely a smaller example -- there has been some disturbance but by all indications it was grazed rather than plowed.
Another restoration biologist noted a decade or so back, as we were starting to see California winemakers buying up similarly hilly lands in the Willamette Valley as a hedge against global warming, that there are likely more prairie remnants that were never characterized or brought into protection, especially up in Polk & Yamhill counties.
Joel Geier