I have app 7 acres, corner lot, B2, adjacent to Walmart. What value should I expect?

Contact a local Commercial Realtor and ask them the question. Too many regional variables for this forum to even remotely get close to an answer for you.
If you don't wish to use a Real Estate Professional, hire an appraiser, to find your value.
In the long run, unless you like the frustration, a local Commercial RE Brokerage is your best bet!

What city and what do the demographics look like? Is it a Super Walmart or an older style? How long has the Walmart been there?

It depends on the location (demographics, traffic counts, near or far corner, and possible uses). Iam happy to help you in the right direction if you want to share some of this info..
Terry

YOUR VALUE IS 3 MILLIONS