Boxing isn't like other sports with regular schedules and events. It is up to a boxer and his management team to decide when and where they will fight. Most trainers, though, don't like to see their fighters go more than 6 months without a fight for fear of losing carefully honed skills.
Boxing requires timing, reflexes, footwork and agility - skills that can degrade when not used for long periods of time. In boxing, it is "use it or lose it".
While the era of the forties and fifties -- when fighters climbed in the ring every month -- are gone forever, fighters at the top level of the sport usually try to fight at least every six months. To go longer risks the dreaded "ring rust" in the return bout.
Four of the men on our list are returning after layoffs beyond six months while three have exceeded that time with no new fight in sight.