Advice needed for Bricks

I just run from 15-30 minutes after the bike on my brick days... it will increase a little but you should get used to running off the bike, even if it is just for 10-15 minutes.

Considering that you're coming back from knee problems, I would be very cautious with bricks. In my opinion, they are most useful as mental training in order to get used to the different feeling that accompanies the first few minutes of running after cycling and being confident in it.

You're not actually building any different physiological adaptations running after cycling. Running is running.

Bricks will be harder on your legs and require more recovery, which is why I'd advocate caution in building them up at all in someone who's coming off knee problems, and especially for sprint distance. (Occasional bricks, still with a short run, can be helpful in nailing down long-course nutrition planning.)

Five minutes or less of running off a cycle no longer than race distance is sufficient. If you find that sensation bothers you, then go ahead and run briefly after these cycles until you're used to the feeling.

Thanks for the advice everyone, the knee is doing fine now, but learned my lesson and will take it slowly with bricks. 

I already tried a 3 miles run one hour after a 18 miles ride and even though my legs felt like lead at first, it went well. I'm more worried right after the ride because just walking feels alien after a long ride.

This is what everyone ^^^is talking about...the transition run is more important than just getting a dead-legged run in. Waiting that long after a bike ride to run, really won't give you that sensation that you're gonna have in a race. I would start running immediately (within 5 minutes) after getting off the bike from your long ride and just get a 5-10 min run in there. Like everyone else said this will help you get used to that "alien" feeling and be able to better overcome that in a race. Don't worry too much about the mileage on that run, it just needs to serve the purpose of transitioning you neuromuscularly and mentally from the bike to the run.

I did bricks for weeks and weeks last year, and it always felt "weird" to run after biking. I never had a problem running fast after biking before or after the brick workouts. This year, I don't do them.

I'm not trying to start a new thread or debate here, but what I'm suggesting is that if you're worried about aggravating an injury, you would most likely be fine to compete in the sprint without having done any bricks.