Campaign level question

I started my game in Prophecies. How will I know when I should move onto Factions or Nightfall? Is there an end quest so to speak in Prophecies or maybe a max level I an achieve in it? Also what about Eye of the North content?

You need to reach the city of Lion's Arch and pick up certain quests to be able to unlock the other continents. You don't need to finish Prophecies to be able to go to the other campaigns. More info here.

You don't HAVE to finish prophecies to move on to the next campaign - you can switch between them whenever you want.

However, there is a 'last' mission to every campaign - it'll stand out by having an ending area where everyone's cheering at you for saving the world, and 15 minutes worth of credits start to scroll up. Unless you're determined to do every side quest, that's the point where I'd move on.

I disagree with the advise to finish a campaign before opening others but want to explain the pro's and cons to it.

Finnish a campaign before opening a new one:
Pro:
1: Story will be told in a logic way
2: Lore wise it is more "right"
3: You can say you did it the old school way and be proud about

Cons:
1: You bought separate campagins including skills and hero's, so you want to use what you bought.
2: You make the game a lot harder for yourself

Travel as soon as possible, to get hero's, higher armor, skills:

Pro:
1: Early access to skills (in factions)
2: Early access to max lvl armor (factions\eotn)
3: Early access to (max lvl) hero's (nightfall\eotn)

Cons:
1: Hard to get track where you are in each campaign when you really start to play them.
2: Cause prophecy's content and EOTN content are on the same map it is hard to see the difference.