I just completed The Blood Stained Stars...which was 52 missions long (there were two branching points...it's possible other choices may change the length). I got no less than THREE new faction missions during the arc.
The arc started with a mission from my L2 agent, which was more like a L1 mission (and from what I've read, the mission is identical regardless of who it comes from). It contained a drop item that had a clue that lead to the arc. In the one I did, there was a note about needing access to all empires...which was quite accurate, as I found myself in all 4 empire areas during the missions (at least I had the Diplomacy skill, or I would have been fried by the Caldari). I don't know if that applies to all arcs.
There are no special rewards, or LP rewards. The ISK reward started at 100k per mission, though it steadily went up, with the last few missions at 250k per mission.
The agents used are for epic arcs only, and will not offer missions if you don't have a current offer from them. However, they are labeled as level 1 quality 0. You are able to get missions from each of the agents you are assigned to go to, even if your standings are below -2 with their faction (though in that case, they tend to give you the "we hate you" messages).
The difficulty of the arc I was started at L1 difficulty, though a couple were nasty (I saw a lot of help requests for Burning Down the Hive, which is one mission where some of the drones scram and web). The missions for each arc are fixed, and will always have the same systems...at least the encounter ones were (and they're all in the same system as the agent giving the mission).
(EDIT)Almost forgot to note: the offers do not expire, nor do the missions. Though there are bonus rewards that will expire. But you can take a break at any time during the arc and return to it when you're ready. I don't recommend doing this with an active mission, since who knows what downtime may do to it
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