"Arcana expire after a few activations and are randomly replaced with a new one!"
Arcana are randomly replaced with new ones after a number of casts, based on the arcana's type. Basic arcana are replaced after 12-18 uses, dashes after 8-13, and standards after 4-7. The new arcana will be chosen from those that you own, and it is possible for an arcana to be replaced with itself. Enhanced arcana will always be replaced with enhanced, and unenhanced with unenhanced. However, Chaos arcana are always enhanced, and will be replaced with another enhanced arcana even if they replaced an unenhanced arcana themselves. Every "click" of an arcana seems to count as a use, including normal dashes while your dash arcana is on cooldown, and arcana that can be activated early such as Arcane Intervention.