I grow Granex 33 Hybrid which is also a sweet (but short-day) onion. When kept outside in the high heat, these began to rot after about 4 to 6 weeks after harvest. (So 4 months for Walla-Walla sounds great to me.) Bringing onions into the A/C would increase shelf life somewhat, but that seems to be unacceptable to my wife. This past summer I had access to a refrigerator that was not being used so I kept a few bowls in that refrig. This seemed to be the best method for keeping the onions. I think I made it to the 4 month point on these.

I also chop and freeze the onions for use in some cooking. This works OK, but obviously it's second to using fresh chopped onions.

Here is a bit of info from the Onions document, www.extension.iastate.edu/Publications/PM1889.pdf Published online by the ISU Horticulture Department.

Harvest bulbs after the leaves fall over and begin to dry.
Cure bulbs in a warm, dry, well ventilated location for 2 to 4 weeks or until the outer scales are papery and thin. Remove the tops one inch above the bulb, Inspect and discard any damaged of decayed bulbs. Bulbs with thin necks should be used quickly as they will not store as well.

Store where air can pass easily in a cool 32F to 40F dry location. Avoid freezing. If it is too warm bulbs will sprout.

Occasionally inspect and remove bulbs that are rotting.

Personally I store them on strings, and use the ones that are looking like they might have issues. They are kept warmer than is perhaps ideal. But the onions I raise are short day so when it gets a bit cooler they sprout. Then I either eat them or replant them.

Bob