What makes a stained glass window last 100 years?

The difference between a window that lasts fifty years and one that lasts over a hundred comes down to craftsmanship at every single stage. Every step builds on the one before it. If the stencil is good, the glass cuts will be accurate. If the glass cuts are accurate, the lead will fit tightly and compactly. If the lead fits well, the solder joints will be stronger. And if the solder joints are strong, the whole window is more structurally sound. Then, if the cement mix is high quality and given enough time to cure properly, you're creating a window that will stand for generations. Skip any of those steps or rush through them, and you're cutting years off the window's life.