American art glass windows were made from 1880 to 1920 and European windows date all the way back to the Middle Ages. Today some of those windows sag, buckle, leak and lose their glass - the lead that holds them together weakens over time and exposure to the elements. Restoring an art glass window - disassembling it, cleaning the glass and replacing the lead that holds it together - is maintenance that needs to be performed every 60 to 100 years. The glass is the artwork - and the lead is what needs to be replaced.
There is a defined conservation process for stained glass windows and restoration can be time-consuming because the process is done completely by hand.
If the lead remains solid, some windows can benefit from simpler repairs - re-puttying and tightening the windows. Depending on the condition of the lead, replacing broken lights (each individual piece of glass is called a light) can be done without a total restoration.