It's the standard size of a vial of insulin - 1000 units, same as the extremely expensive insulins referenced in the article and by other people in this thread. How long it lasts depends on how much you need to use, which can vary widely.
Just for clarification: Does Walmart sell an Insulin approved for human use? Animal insulin has been used for human use, and human insulin for use in pets...
Insulin's amino acid sequence varies a little by species, but as far as I recall most mammal's insulins are interchangeable.
Novolin N is human insulin. Walmart pharmacies likely sell more than one brand/type, this was just the one my vet prescribed (although a formal prescription isn't necessary, you just have to ask the pharmacist for it). It was about $25, so I assumed it was the same stuff the parent comment was talking about.
You can get by if you're desperate, but the 28 days isn't an underestimate. The dropoff is pretty fast after the four week mark. I don't understand why, but that's been my personal experience.