You can only do a Timed Review for a skill after getting Crown Level 5?
Really Duolingo? This was one of my favorite ways to practice skills, but now you have to repeat the same exercise 40+ times before being able to do a full overview of 1 skill! I kind of like the crown things, but this is just sort of disappointing.
General "Strengthen" practice, timed or not timed, is available, as lindakanga noted, from the main screen. If you are interested in having it help you strengthen the lessons, as marked by Duolingo, it seems to have no effect.
Timed Practice of individual skills is no longer available until, it is said, level 5 is completed. It was by far my favorite way to practice, too.