Thanks for forcing me to think of this.
What is consistent? I guess it depends on the publisher. For Highmoon Games, consistent might mean 5 copies a month (as opposed to the trickle-in of a sale here, then another in 2 weeks, then 1 month, etc.). For a company the size of Evil Hat, perhaps it is 20-30 copies (what's the sales pattern for SotC still now? I think that defines it). For WotC, it'd be a couple hundred, I would think. Can we say a 5-10% of initial month sales numbers fully realizing that no such study or comparison has actually ever been done?
But like I said, there's another element to the "evergreen" adjective that to me, and especially when I was working sales in a hobby/gaming shop, denoted enthusiasm generation, and that is a lot harder to quantify.