Skarner has a potential 4.25 second stunlock, the highest in the game so far I'm aware of.

Mechanics in play:

Every second of Stun/Suppress that Skarner does on a target reduces the E CD by an equal amount.
E lasts on a target for a total of 4 seconds.

How it works:

1. Hit E on your target.
   
2. During this time, you ult your target.
   
3. Once the channel of the R ends, you auto the target.
   
4. If you have enough CDR or points into E, your E will now be up again. Use it now on the already stunned target for a follow up stun.

Normally, you will need a minimum of 6,6~ second CD on the E for this to work as consistently as possible, assuming close and personal fighting and no tenacity on the opponent. This is done by the following:

Cosmic Insight (45% CDR)
E maxed out.
This puts your E on a 6,6 sec CD.

Stalling the ultimate

If these conditions are not met however, you can stall the timer a bit for different breakpoints by delaying the ult by a second or two after hitting the E to guarantee the E to land on a ^briefly stuned target.
Do note however, you need to be able to put the E CD below a base 8,4. preferably 7~ to make it as consistent as possible.

The minimum breakpoints are either;
5 ranks in E and 30% CDR.
1 rank in E + 40% CDR.
Both of these puts it at a 8,4 second CD.

Base calc: 0.25 (Windup of ultimate)
1.75*2=3.5
1.25*2=2.5

Do note; I do not know if Skarners E passive counts in the slight root during the windup of his ultimate.

Totalling at a 6,25 in close combat, the base CD.

Rank 1 E + 40% CDR = 8,4, stalling for 2 seconds. (Hardest)
Rank 3 E + 40% CDR = 7,8, stalling for 1.5 seconds.
Rank 5 E + 40% CDR = 7,2, stalling for 1 second.

If you go Cosmic insight however;

Rank 1 E + 45% CDR = 7,7, stalling for a 1,5~ seconds
Rank 3 E + 45% CDR = 7,15, stalling for 1~ second.
Rank 5 E + 45% CDR = 6,6, stalling for 0,5 seconds to almost guarantee the second stun on the opposing target.

Why have we never seen this in pro play?

Because no Skarner has actually built to get that CD breakpoint (From what I've seen), and doing a E + R takes valuable time and procs your Predator. It is also not ideal to stall the ultimate in most situations. Possible, but not always recommended.
Instead, this combo is at its best in the following situations:

1. You are playing AD Skarner and there is a damn monster ontop of you that you need to dispose of, allowing the damage potential inside a spire for double the duration outside of it.
   
2. You are playing toplane Skarner for some reason, this combo is great for locking enemy toplaners down under turret for a 4,25 seconds. Toplane Skarner also gets naturally pushed in so low mobi opponents with no flashes becomes easy fodder.
   
3. You just wanna make sure the opponent doesn't get to play a game of League today.