As stated above...it really does depend....
A counter attack is always a good place to have a go as well as after a prime. A lull in the pace can also be a good time, but that really depends on how the race has been leading up to that, have you been full gas for the last 15 minutes or has the race been pretty comfortable.
Attacking at a crosswind section can also give you a good opportunity to establish a break.
Don't kill yourself when you make the attack because otherwise a mile later you will probably be reeled in by the group and spit out the back.
A good rule of thumb a lot of others will tell you is that if you are hurting, there is a good chance others in the group are hurting and that is a good time to attack because others will be less inclined to chase.