Time Card Question

[Niwashi]

Is there a length of time after which your chars will be deleted if you stay away for too long?

No. Characters (and all their items, etc) are kept permanently in hopes that anyone who left the game might come back someday. The only thing you could lose is a house if you get one. Housing plots are limited, so those can be reclaimed by the system if not used. (The reclamation feature is currently halted, but scheduled to start again with the 3.4 update.)