These quests have no limit on the number of times you can finish them. The only limit on these quests tends to be a level limit, where the player can no longer receive the quest if their level is beyond a certain point. The reward for these quests is the same each time you finish them. City Writs fall under this category, as do certain item-drop quests that grant access to a special instanced zone. These quests will always show with the (repeatable) tag on your completed quests tab.