As a general rule, the longer the event, the shorter the warmup (except for handicaps!). 

Long road races tend to start off fairly easily, and often the most important part of a long race warmup is to ride the finish of the race (if possible) to see where the finish line is, if there's any hazards and so on.  Your warmup for a road race is usually 10-20 minutes of E1 with maybe a couple of short high cadence efforts to bring your heartrate up into E3 for a minute or two.

Shorter races, such as criteriums, or longer, but more intense races, such as handicaps and time trials, require a more intense warmup - you want your body to be used to working at the intensity required for the race, not to be shocked by it.  As for road races, you want to have a look at the finish of the course if you can, which with criteriums is almost always the case, but may not be for longer races.

A good warmup for a crit or h'cap can consist of :

• 10-15 minutes of E1 (or, ride to the event!)
• Two or Three short E2 efforts (2-4 mins E2), two or three high cadence (10s HCLR) efforts
  
• One or two short E3 efforts of 2-4 minutes with 2-4 mins E1 between efforts if you do multiple efforts

This will take around 30 minutes or so to do properly.

You want to time your warmup so that you start your race as close to the end of your E3 effort as possible - it is a good idea to do this on rollers or a spin trainer for important events where you're not sure how long you might have to spend waiting for your start, as you don't want to go to the trouble of warming up and then getting cold while waiting for the race to start!

It's important to use the above merely as a guide, as for every rider what works best for you will vary, and we'd like you to discuss your warmup with your coach, especially for important events!