I have two batches of chicks, 3 weeks apart. I put in roosts for them when the oldest group was around 6 weeks old. I put them in their permanent spots, with one higher than another by about a foot. At around 7 weeks some of the older ones started showing interest in them, and now at 11 weeks all the older ones roost at night on the high roosts, just like grown birds. My younger group of chicks is just hitting 8 weeks, and they're still sleeping on the floor.
I second the "as soon as possible." I start with a short roost (about 6 inches long) with a double threaded screw thing in one end. It has a nut and washer and attaches right to the side of the wire. If you don't want to make one, you can probably find them at the pet shop - but parrot sized - chicks have big feet! It is only a couple of inches off of the floor. Once they start using it, I put the full bar in. Ours were all lined up on the roost at about two weeks old! I've raised it to about six inches now (at three weeks)...except for the occasional ***thud*** from one who forgets to hold its head up as it falls asleep (and subsequently falls off) they're all doing great with it. The biggest thing is not to put too small of a diameter in there...Go for it! It is so cute to see them roosting!
I have a small box (like 6x6x6") in there on its side so they can get out of the heat or light if they feel like it, and a small piece of trim ~3/4" across diagonal between the feed and the box. I use medical tape or electrical tape to attach it to the box and the feeder. It's not perfectly level, but they don't care.
The only caveat is that the more vigorous kids will get to roosting on the top and eventually leave the tub. I have them in a closed bedroom bath/closet, so it's okay- but I have to have a box on the outside so they can get back in the tub, and I have to make sure our toilet lid is always down...
Mine rarely have roosted before 12-14weeks. I don't really see it as a problem. Now I just wait until I move them out to the coop at 8-12weeks and don't bother with a roost in the brooder. No one has ever used it.