They didn't fight for a year straight though. They took breaks in between then. It actually depends on who is fighting who and how much energy they put out. Vegeta fought Frieza and out matched badly and wasted all his stamina focusing on energy blasts, and was a sitting duck afterward. Kid Buu, and probably the androids outside of Cell could fight for extremely long periods of time due to their physical make-up.
Android 16, 17 & 18 can fight forever due to their endless supply of energy. Android 20 & 19 could potentially do the same, under specific conditions. (A stream of energy to absorb) Buu can fight forever due to his magical composition, he never tires. He slept for an entire second as fat buu, although i consider this an act of comedy from the author.