How long should I level in the dunes for? I'm at 17 now. Also were should I go when I'm done with the Dunes?

Thanks.

Generally, you level at the Dunes till about level 20 or so. Before you reach that point though, once you reach level 18 for the first time, you should get your Sub Job. To do this, speak with an elvaan NPC named Isciao, on the docks in Selbina. He'll ask for three items: a Crab Apron, a Damselfly Worm, and a Magicked Skull. These three items are dropped by monsters in the Dunes.

Crab Aprons are dropped by Snippers, Damselfly Worms by appropriately enough, Damselflies, and finally, Magicked Skulls are dropped by Ghouls.

Once you have these items, return to Isciao and trade them to him (he'll ask for a particular item one at a time). Once you give him all three items, he'll give you the ability to equip a Sub-Job, a very important ability to have.

Once you gain your subjob, it's best to return to your home city and start levelling the Job you choose to equip as your sub (your sub job doesn't gain experience otherwise). It also is limited to being half the level of your current Job, so once you reach the maximum level (75), your sub will be 37.

Therefore, at the very least, level your sub job to half the level of your current Job, or better yet, to level 37. I know it's time consuming, but it is worth it, as your sub job can add major bonuses to your current Job, provided you keep it levelled and choose it wisely.

Once you reach level 20, the next area to level is Qufim Island, located next to the Grand Duchy of Jeuno. This is the place to level between levels 20 to 25. To get here, head to the city of Jeuno. Jeuno can be reached by the following routes:

From Windurst:

East Sarutabaruta>Tahrongi Canyon>Merithartaud Mountains>Saruomuge Champaign>Port Jeuno

From Bastok:

South Gustaberg>North Gustaberg>Khonstchat Highlands>Pashhow Marshlands>Rolanberry Fields>Lower Jeuno

From San d'Oria:

West Ronfaure, La Theine Plateau>Jugner Forest>Batallia Downs>Upper Jeuno

Be very careful when heading to Jeuno on foot, especially in the areas before Jeuno, as the monsters here are dangerous at level 20. If you have Sneak or Silent Oil, use it here.

Qufim Island is reached via a tunnel from Port Jeuno (to avoid agro, take Silent Oil, or use the spell Sneak if you're WHM or RDM). The Weapons found in the tunnel agro to magic though, so once again, be careful.

Before you continue levelling though, it would be best to take a break and get your Chocobo License. To do this, head to the Chocobo Stables in Upper Jeuno, and speak to the manager, Brutus, He'll ask you to feed the sick chocobo. Feed it four Gausebit Grasses (bought at the Auction House - you can only feed the sick chocobo once per Vana'diel Day though), and it will recover. Once this is done, you'll recieve your Chocobo License, allowing you to ride chocobos from any stable or handler, making long trips though dangerous terrian a lot easier!

Hopefully, this hasn't seemed too daunting..