Getting to Ishgard

So in order to unlock the expansion classes like Dark Knight and Machinist you have to first reach Ishgard and complete a series of end game quest, correct? Just how much are you suppose to complete and how long does it take.

You have to complete the whole ARR storyline up till patch 2.55. Last quest is Before the Dawn if I recall recorrectly.
It's not just getting to level 50. At 50 you have a lot of stuff to do. Took me a few weeks playing very casually next to my RL job.

As Elysaine mentioned, you need to have completed the entire '2.X' series storyline to enter Ishgard (and thus play the expansion content), as the storyline leads to there. The 2.X story consists of the original ARR story that ends at the quest The Ultimate Weapon, and which then was continued through five separate patch storylines - 2.1 A Realm Awoken, 2.2 Through the Maelstrom, 2.3 Defenders of Eorzea, 2.4 Dreams of Ice, and finally 2.5 Before the Fall.

Each storyline continues where the previous one left off, making it one very long continuous story arc. As Elysaine pointed out, the very final one that the Heavensward story then picks up from (which grants access to Ishgard as a part of it) is the patch 2.55 story quest Before the Dawn. It will probably take a couple of weeks to get through all this story (and it's connected dungeons and trials that you have to do as a part of it).

Don't despair though! Take your time going through it and enjoy the story at your pace (in other words, don't let players boss you into 'skipping the cutscenes', particularly with Castrum Meridanium and the Praetorium.) If you're curious as to what you have to expect, this is a list of all the current story quests in the game: http://ffxiv.gamerescape.com/wiki/Ca...Scenario_Quest (it should go without saying but there are possible spoilers there).

Now before you cry and swear at Square Enix, they did specifically mentioned that the main story up till patch 2.55 needs to be done before getting into Ishgard and all that new stuff. There has been several threads about this and none too happy about it though i would strongly advice you to be patient and ride through the story and progress till the Expansion as it is the way it is intended to be played. It's a story based MMO after all. There will be a few annoying moments where you are required to complete certain Guildhest quests, those i will agree that SE should streamline to improve the pace and ease of getting through the long list of quests but none takes too long to be done as i find that queueing in Duty Finder to get a group going doesn't take more than 10 minutes to form these days. If you can afford to play for more than 5 hours a day, it shouldn't take more than 3 days to get to Heavensward. I took my time and roughly reached Ishgard in a week playing 1.5 hours a day.

While you're going through the patch storylines, the story gives you cool equipment with high item levels and good materia as compensation.

Granted that you can't enter Ishgard before finishing the full main quest story line, but can you start earning experience points above Level 50 before that point if you have the Heavensward expansions registered?

If you have purchased Heavensward, you will get XP at level 50 for killing mobs and doing quests. My DRG is almost level 52 just from the quests after turning 50 and doing some daily quests.

That's what I was trying to figure out- all this experience I can't use (without the expansion) flying past as I work my way through the MSQ past 2.0. Getting experience now on my BLM and SMN would make it worth it to purchase Heavensward now while I work through the rest of the main story quest.