Some short testing in melee indicates that when it procs off melee attacks, there is no internal cooldown. I observed frequent and multiple refreshes of the proc prior to its expiration. This seems consistent with others' findings.
Also, in melee, crit damage benefits from
[Relentless Earthstorm Diamond]. Every crit was for 232 damage (1.545*base) instead of just 225 damage. And it procced from Lightning Shield damage, Windfury attack, Flametongue attack.
In casting, the minimum time period between two procs while spamming rank 1 Lightning Bolt was 25.5 seconds. It never refreshed.
I never got a crit while casting and wearing enhancement gear. I frequently crit while meleeing in enhancement gear. A more thorough test would have to be performed to reach a conclusion on when it uses melee crit rate vs casting crit rate.
I'll venture to guess Shaman can get a lot out of this relative to other classes, assuming the melee mechanics and casting mechanics are unlinked. The suggested 50% proc rate with 25s or so ICD on spells could mean decent proc rate from shocks on top of the PPM from enhancement melee attacks.