I like about 25' total to get ready for my Test.

Usually goes like this:

5' easy
5' ramping up into tempo wattage, gently, by feel.
1'easy
30" @ 90-100%
1-2' easy
60" @ 100%
2-3' easy
90' @ 100-105%
3-5' easy

GO!!

more than anything I want to be mentally confident before starting my test, ready to go but not demoralized by my warm up. this seems to ease me into it quite well.

-L