What makes you think Ginsberg is likely to retire in 2-3 years ?
I think she and Stevens will retire the moment there's a liberal President. Three years is therefore the minimum.
Anything before that would take a miracle (no, I'm not praying for someone's death)... though it isn't out of the question. Stevens is only a couple years younger than the record a may not make it, and Ginsburg has had intermittent health issues.
But there's no way that they both make it seven more years. It makes the next presidential election massively important. If we win that one... the court changes for decades.