Description makes all the difference for a reader, as far as a full connection with your work is concerned. Too much description has an undesirable affect - it will turn the reader off. It's actually easy, once your typing is fast, to ramble on and on; but the reader seldom appreciates this more insidious form of lazy writing. However, too little description means the reader finds it difficult to connect with the characters and scenes you are writing. They might not be able to put their finger on it, but they will find it hard to tell certain things about a character that you take for granted. You might be aware that a particular character walks slowly and dresses like a dandy - but your reader won't have a clue. In that duelling scene, you might know that there is a desk up against the left wall, and that's exactly what will sabotage the opponent's fight - but does your reader know?
1. Description should have a purpose.
Never just slot in description for no reason. Some of the reasons you might use description will be atmosphere, increased understanding of a character, increased understanding of a scene. But always make sure you know, and you're not just slotting it in because you're such a great writer.
a. Describe surroundings before a fight scene.
If you're going to have a big fight scene, the reader must know where the important features of the landscape/room are. If they don't, the fight scene won't make a lot of sense.
b. Describe surroundings to reflect a mood.
If your main character is depressed, angry, or elated, an easy way to show the reader this is to describe things from the character's perspective. Maybe the sky - blue and glaring - looks like it is going to eat them up. Maybe the sun is sending reflections off the water just to duplicate its wonders. Whatever it is, description is a great way to show the character's feelings without resorting to the obvious: "He is sad". "He is happy".
2. Description should be well-placed.
This is a tough thing for writers, but description should be interspersed well in the writing. It should never come in huge blocks repeatedly, as this is a huge turn-off for the readers. Description, in my opinion, is the background for action. Don't let the background become the foreground. One of my favourite devices is to start a scene with someone talking or doing something. Then I say in the next paragraph: "They were pacing in the well-lit sitting room, so-and-so holding a dagger in their hands. It was night, but the candelabra on the centre table, etc, etc". Another device that is well-known is to place parts of a character's description in dialogue. "'I have no idea what you're talking about,' she said, whirling around indignantly, her white-blonde hair whipping about her head." Okay, it feels a little contrived, but if it's clever, you're reader won't even know it's happened.
3. Lastly, description should be just enough and no more.
Once you're certain the reader can see things clearly enough for the purpose of the scene you are writing, don't go on. Leave it. If there's other stuff you want to add about the colour of someone's eyes, save it for later - maybe in a dialogue, when they're glaring at someone. But never stretch the reader's patience.
Enough description is always determined by the scene you are writing.