4000 rounds in 9 weeks! Holy crap!
I don't think you're going to fine a definitive answer of, "The XD will last ______ rounds before it dies." There are guys, some here, and more on XD Talk, that have waaaaaaay more rounds through their guns than yours and they're still working. Maintain it properly and it will last.
FYI, the XD has a striker, not a firing pin.