You want to take your soil sample to the approximate depth you will be plowing. 7 inches would be a good depth if you are not sure just how deep you are plowing. Haystack is right on about taking a lot of cores, and mix the sample well in a bucket before sending it in for analysis. There should be plenty of time to take a soil sample, get the results, and spread lime this spring. You can always plant a plot to your current soil pH this year, a spring and a fall plot, and/or plant something more permanent next year. There are many good choices that will tolerate lower pH soil.