There are 2 kinds of bricks that I usually have people do. One is the dreaded long brick that is meant to really let you get a feel for what your legs are going to be like off the bike on race day. On that brick I have people just run after their longest ride of the week but only for 20 to 30 min. I only have them do this for the last couple of months going into a race. If you can get in 3 of these workouts, you're doing pretty well.
The other brick is meant more as a neuromuscular retraining workout and for practice of the transitions. On this one I'll have people transition to the ride (timed), ride 3 to 5 miles hard, transition to run (also timed) and run 1 mile hard. rest a couple of minutes and repeat about 2 to 3 times. It makes for a fun mid week workout. This is a good one to do in the last 2 weeks prior to starting a taper and to even do a shorter version as a workout during your taper before a race.
Hope that helps.
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