When a restoration like a crown fails (breaks, fractures, etc.), I like to be proactive rather than reactive. Meaning, I would recommend replacing it as soon as possible. If the crown is made up of a substructure of metal (gold, etc.) or zirconia, often times the underlying tooth structure is sealed and safe for a while even though the overlying porcelain may have fractured. The fractured area usually leads to an open contact between the adjacent teeth, which can result in food impaction and ultimately recurrent decay or periodontal inflammation. I certainly do not recommend waiting until either of these scenarios occurs. If the underlying tooth is sealed and contacts between the adjacent teeth are closed, you may be safe for a while.
Crowns can break for a variety of reasons, but most commonly they fracture due to excessive forces being applied and the material fails.