Short naps??

How long should my 3-month-old be napping? We try to follow an Eat/Play/Sleep routine, and he has no trouble falling asleep, but he NEVER naps longer than 30 minutes. I can almost set a 30 minute timer, they're that consistent. Shouldn't he be sleeping longer than that?? He gets so cranky by the afternoon. And I know naps are important for their development. Any tips?