All pieces of equipment, regardless of their quality, have a base durability of 30.
You can increase this number to 40 by researching all the "Increased equipment durability" technologies in the Blacksmithing branch.
When an item's durability reaches zero, you can repair the item using Silver. After each repair, the item's maximum durability decreases by 1 (i.e., after 30 marches, the durability of your equipment will decrease to 0/30; when you repair it, its durability will be 29/29).
The higher the rarity of a piece of gear, the more it costs to repair it.
Once an item's durability has reached 0/1, you can no longer equip or repair it. However, you will be able to dismantle it in order to extract one random material that was used in the crafting process.