When a collar is appropriate depends entirely up to the couple involved. However, there are a few fairly general guidelines with them. Typically an official collaring, involving a collaring ceremony, is similar to the idea of a wedding where the couple is making a serious committment to each other. However, reaching a point where a submissive/slave is ready to be collared can take time and training. Because of this, often a "training collar" or "collar of consideration" is worn by the sub. This is an outward mark to indicate to the community that the sub/slave in question is spoken for and in a committed relationship and so is not 'up for grabs'. A collar is a serious step and not something to be taken lightly or given out too quickly. It must be earned by both parties through committed exploration into developing a bond between the sub/slave and Dominant.
You might want to read through this thread for more detailed information: http://www.bdsmlibrary.com/forums/showthread.php?t=9948