You are limited to 30 total dungeon clears every week per character. Once you hit level 50, these clears are reset, meaning that you won’t be penalized for doing the dungeons before level 50 as part of the story.

In addition to the weekly 30 dungeon limit, you are also limited to 10 dungeons per day. Keep in mind these are limited per character, meaning that each character can run 10 dungeons a day, and 30 dungeons a week. This is why having multiple characters is great, especially for the extremely valuable Onyx Crystals and Chaos Onyx Crystals from dismantling dungeon equipment that you don’t need and can’t sell.

When you want to start doing dungeons at 50, you have two options. First is to start doing the 1500 GS level 50 dungeons in order to get a chance at an Epic equipment drop. If you don't have enough GS to do the 1500 GS dungeons, try using your +7 scrolls from leveling up on your gear that are less than level 50, or spend a few mesos and buy level 50 blue gear from the Black Market. If you want to go straight to the 2100 GS hard dungeons, you will need at least one epic gear. The weapon and suit give the most GS, but are the most expensive, and you may also have to enchant a few items to meet the requirement. You can also try to buy a cheaper epic equipment (not Pyrros gloves since it barely gives GS), and then enchant your level 50 blue equipment to barely reach 2100 GS. When doing this method with the enchanting, you will have to also do a trick where you equip a blue main-hand weapon like a scepter, and then equip a throwing star or dagger in your offhand (not codex or shield because they don't give GS) before entering a 2100 dungeon. This will allow you to enter the dungeon, and then you can swap to your normal weapons. Enchanting your weapons and suit/top/bottom will give you the most GS per enchantment. Here is an example of me getting enough GS with an epic accessory: GearScore