How in depth should a general campaign setting be?

I have a big plot but im wondering if its too much or maybe from a different perspective their may be alot of holes in it. Basically is their some kind of guide line that will be nice to go by when creating a homebrew campaign.

There probably is but your own instincts will do you well. Look at it and ask yourself if the players care to know this or is it justification to your mind why something happens. Make general notes about this so that you have reference anything in detail wont matter unless the players feel they need it. Second thing is that plan on the players breaking or not doing what you thought of. Some of the best moments in the game is when players go and do something unexpected so dont create anything based on players need to do or know. You will put yourself in a corner if the characters have to do sometjing to get a clie or be able to know where to move on to the next part of the adventure. This also applies to the campaign setting. Make it in general with enough detail to entice the players. Sorry for the rambling. Others will be be to voice their thoughts on this also. Just remember to these are opinions and suggestions. Use what you want or like and ignore the rest.

Moved to Off-topic - Gauss

There are a couple of guidelines out there that you can use but I would not suggest using them. Personally doing so creates a bland campaign in my mind. Your cities, kingdoms etc. all tend to be uniform and no world or plot should be uniform. When creating a world or even a plot/adventure I tend to keep in mind a few things. 1) Always keep your audience in mind. While you want to create a world or plot that you are going to have fun with when it comes down to it, the players are what drive a campaign and as long as they are having a blast then you will as well. Each group tends to have things they like; themes, styles, rp to combat ratio etc. Knowing what a group wants to get out of the game is paramount. I generally do this by either a) talking to my players before hand to get a feel for what they like in games or are looking to get out of one. or b) running a introduction storyarc that has a mix of theme's, styles, and combat to rp ratio and merely observing what gets their attention. 2) You don't have to be incredibly detailed from the get go. As a campaign goes on you can always expand on stuff as need be. This allows you a lot of freedoms as not only do you not have to come up with everything right away but allows you to customize your campaign to your players needs. As Metroknight mentioned players tend to do the exact opposite of what you are expecting or plan for, so keeping your plot and campaign rather loose and fluid allows you to compensate for any unexpected turns. Now doing it this way does require you to think on your feet a little more so planning out a few scenario's as a backup could help. Alright well I am going to end it here lest I start to get preachy. As Metroknight said, use what you want and discard the rest.

Phisto has the gist of it. Keep your plots broad and openended. Build only what you need for the game. Make notes otherwise. If the player characters are not going to meet the 3rd cousin on the kings mother's side then there is not reason to stat him up and build a sheet. Make a quick note of who he is, his personality, and how he might be involved in the game. This is just on the off chance the pc's decide to ride off to the distant city where this 3rd cousin is and happens to meet him. Believe me, you might put out a juicy tidbit of a rumor for the pc's to swallow and they might ignore it and go for the 3 week old hunk of ironrations then take off in the opposite direction you wanted them to go. Almost every gm has had that happen to them at least once.

Generally in my opinion, I find it best to work with existing worlds when it comes to creating campaigns with specific goals in mind.  Mainly because it's easier to pull alot of the side story/side quest details out of published materials then it is to make an entire setting from scratch (not that that's a bad idea, just make sure you know what you're getting yourself into first :P).  In the case of creating an entirely new campaign setting, start with the big picture and work your way down, keep it simple to start, then expand upon it as you see fit.  If you want to make your life easier, do things like creating random encounter tables for different regions if its a sandboxy style campaign (or you plan on having random encounters :P).  Metro hit it on the head though regarding keeping things broad and open ended plot wise, let the players actions hash out the minute details, it saves you alot of work and generally makes things more interesting.