It will take time before it is 1% or below. My guess would be 8 years, people tend to hold on to old stuff way to long. This is especially true for people that don't know computers and doesn't have a legal copy of Windows, and people that don't know computers and have automatic updates turned off.
But why wait until it is 1% or below? It depends on who you want to browse your site, if you want the categories of people mentioned above you have to wait. If not you can disregard it much earlier. I would say that you can disregard it when it is below 5%, I think that should be in about 3 years.