How high should I level up my hero and is there a maximum for heroes and when should I move on to the..

Heroes can be leveled up with Exp to a maximum of 50. Leveling happens on its own by using the hero. Or with the Exp items. The level can help with some heroes to improve them but is mainly just needed for unlocking skills. Don't worry about reaching a high level for any particular heroes unless you are able to unlock all of their skills. It is the skills that will give the most benefit.