What is the normal duration of a GD?
A GD is generally of 15-20 minutes duration.
How many panel members are there to evaluate?
There are usually 3-4 panel members to evaluate.
Is there time given for preparation after the topic is given and before starting the GD?
Usually some time (2-5 minutes) is given to collect one's thoughts, but there could be instances when this does not happen, so it is best not to bank on this.
Should I address the panel or the group members?
Don't ever make the mistake of addressing the panel members. The GD is between you and the other members, not the panel members. You must avoid even looking at the panel members while the GD is in progress. Just ignore their existence.
What is the seating arrangement like?
It could be semi-circular, or circular, or seating along side a rectangular table, depending upon the venue. It is best not to bother about trivial issues like this, which you have no control over.
How should I address the other group members?
If you are initiating the discussion, you could do so by collectively addressing the group as "Friends". Subsequently, you could use names (if the group has had a round of self-introduction prior to starting the discussion and you remember the names) or simply use pronouns like "he" or "she".
Suppose I have a lot to say on the topic, should I say all of it?
You would not be looked upon favorably if you kept speaking all the time and did not listen to anyone else. Contrary to the misconception, the person who talks the most is not necessarily the one who is judged the best. The quality and not the quantity of your contribution is the success factor.
Should I encourage others to speak up?
Do not directly put someone who is consistently silent on the spot by asking him/her to speak up. If someone has been trying to speak and has a good point but is cut off constantly, you may encourage him/her to continue with her point as you would like to hear her out.
Are the group members supposed to keep track of the time or will the panel keep track?
It would be good if you are conscious of the time, but not to the point of getting so distracted looking at your watch that you do not contribute to the discussion.