Longest possible combined CC duration In League of Legends?

According to the wiki, there's 4 types of hard crowd control: airborne, forced action, stun, and suppression.

Assuming max CDR and level 18:

                          cost per      cost/
Champ        cc  secs cd  stun     mp5  mp10
--------------------------------------------------
Brand        q+w 2    3.6 140      18.2 3.9
Thresh       q   1.5  4.2 80       19.6 2.2
Xerath       e   2.25 6.6 80       19.6 2.2
Fiddlesticks q   2.25 6.6 65       19.6 1.8
Elise        e   2    6   50       19.6 1.4

So, if each champ chains their cc once, the target will be stunned for 10 seconds. After 10 seconds, all of the abilities will be off cooldown, so we only need to worry about mana. Dividing the champ's stun cost by the champ's MP10 gives us the percentage mana regen needed to recoup the stun's cost in the 10 second window. Brand's is the worst, at 3.9. Base mana regen + Morellonomicon + Mikael's crucible + Talisman gives brand 400%, which covers the mana needed.

Therefore, it is possible to infinitely chain hard cc on a single champ, given the right champs and items.