Leona is one of my favorite supports, and one I think is underrated a little. One reason is her damage output with her passive and absolutely no offensive items. Another is the strong survivability boost from W. But the main reason is her low cooldown, long range Crowd Control.
Oh, and she starts with a free Bead, practically (9 base hp5).
First, I'm counting her root from E as CC, and estimating its duration at 0.75 seconds. Assuming that's true, with 0% CDR, ignoring her R, she already has 26.2% uptime. Add 20% CDR, it rises to 32.7%, and at 40% CDR it's 43.7%. I think that's fairly impressive. And that's assuming an infinitely long engagement.
Lets look at what she does in a more realistic timeframe, say, 10 seconds, using the regular E-Q-R combo. That's 2 E's, 2 Qs and 1 R, 15% root uptime, 37.5% stun uptime, 52.5% total CC uptime (4.75 seconds free of CC).
At 20%, you can get this in over 8.5 seconds for 61.8% up time (3.1 seconds between the two rounds of CC), and at 40%, it happens over 7 seconds for 75% up time (1.7 seconds between rounds).
Of course these calculations aren't exactly directly LCS relevant, considering how little time supports spend at level 18 in LCS matches. And they rarely have the chance to get much CDR. But from a soloQ perspective, these are relevant numbers.
As always; numbers have been rounded to make them a little easier on the eyes.