Arenas wasn't tried; he entered into a plea bargain with prosecutors, which left his sentence open-ended. Delonte still maintains his innocence, and as of yet, hasn't entered a plea. Another difference is that Gilbert was in Federal court (having been arrested in D.C.), whereas Delonte was arrested in Maryland, and will be tried in state court. Each system has different docketing rules, discovery schedules, and court calendars.
Typically, it can take in excess of a year if a case goes to trial. My guess would be that neither the prosecution nor defense is all that anxious for a speedy resolution in the Delonte case.