I have no idea of the numbers but I would say if your company provides support for your products running on Windows 2000 then you would continue testing. At the point where your company drops support for the products running on Windows 2000 then you would stop testing.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison
Let me rephrase. My company is considering dropping support of win2k for our product. This decision will be based on the number of people that are still using win2k in the USA. We are finding that it is a difficult piece of information to obtain though.
We usually dropped official testing for Windows platforms when Microsoft did, Support still kept a machine around for checking and in the rare case we might fix a bug that way we still had an environment to check it on. Otherwise unless your Sales force has an idea of usage, its all pretty much relative.
Nothing learns better than experience.
"So as I struggle with this issue I am confronted with the reality that noting is perfect."
Funny thing that, we stopped testing on win2000 a couple of versions ago but to mitigate that we still have a Beta Site that is on Win2000 so that if anything does come up, hopefully they will pick it up before release...just such a case has just happened as we have upgraded to VC++ 2005 compiler which has some more stringent checking and has raised some issues with our code on Win2000, this has been picked up in Beta and now fixed. I think that it would be fair to assume a number of users are still on win2000 (especially business users), why would they upgrade?