in my training, i never do a brick longer than 30 minutes. my coach believes that the purpose of a brick is to get your legs used to the motion of running after being on the bike only - so i usually do two a week on my build weeks and none during my recovery weeks.

everyone is different though! some coaches believe in 3 hr bikes followed by 10 mile runs. i've only done that once - during my first training cycle for imfl. in preparation for imcda, the longest i did was 30 minutes and i felt completely prepared.