most our skills are 1.5 seconds or less cast time, math?
True but depends on what you're casting. The following are some of the top DPS spells for Conjuror that we cast.
At 100%+ Casting Speed and Reuse Speed:
Consuming Inferno - Cast Time 2.5 seconds, Recast 35 seconds.
Communion - Cast Time 2 Seconds, Recast 1 Minute
Aqueous Swarm - Cast Time 1 Second, Recast 30 seconds.
Roaring Flames - Cast Time 1.5 Seconds, Recast 30 Seconds
Vampire Bats - Cast Time 1 Second, Recast 6 Seconds
What I see as the biggest problem with the 7 second (Actual is 6.7 seconds) window is the recast of most of these abilities. I suspect this is the same problem for other DPS classes when/if they use our shards for the CBOC and not power. Timing wise I can get off 2-3 of these abilities when I cast my own shard storm/use a shard. Coordinating this with an entire group is near impossible in the 7 second window except at the very beginning of a fight due to the recast times. I have never even attempted to get my group to coordinate using their shards all at once, and no one wants too anyway. Generally, people in group/raid wait to use a shard/heart when they need it for power, not because of the CBOC, or fervor in the case of hearts. The CBOC when group members use their shard is nice when it actually lines up with what I'm casting, but it rarely happens from what I have seen.
The shard itself says 4.2 seconds for Stormfront but testing it shows the full 6.7 seconds. The effect works on the entire group. I believe the developers are not just looking at summoner cast/recast times. They are looking at other DPS such as sorcerers and scouts too when these are used. I'm not sure how much other DPS classes benefit from these shards, but I doubt it's much if they only use it for power and don't time it with their big hitting abilities.
Slightly more time would give a bit more wiggle room when it comes to spells/abilities available to use with the buff and may make people consider using them more so groups actually benefit from hearts/shard.