I'd say they need to rejuvenate or revitalize themselves from time to time.
What form that takes...I wouldn't know. Maybe after they mate or spawn?
Perhaps they indulge in a reverie like elves, but one filled with the beholder's favorite moments of inflicting terror, madness and mayhem on other, lesser beings.
I think it would be awesome to put a group of adventurers in the middle of a hive of beholders, all dreaming mad dreams, making scary noises and drooling in somnolent glee.