SD has a 6 sec duration and a 9 sec recharge time. Assuming a Guardian has T15 4set, and either Berserk or Son of Ursoc is popped, it is very safe to assume 100% rage for the next ~10 secs. So:
6x3 = 18. By popping 3 Savage Defenses back to back, you get 18 full seconds of dodge. 9 sec CD
At 9 secs left, your 1st charge of SD will be back. Popping it will bring you to 15 secs. Another 9 secCD
At 6 secs left, your 2nd charge of SD will be back. Popping it will bring you to 12 secs. Another 9 secCD
At 3 secs left, your 3rd charge of SD will be back. Popping it will bring you to 9 secs. Another 9 sec CD.
At 0 secs left, your 4th charge of SD will be back. Popping it will bring you to 6 secs. Another 9 sec CD.
At this point, SD will reach 0 secs when you've still got 3 secs left on the CD. When those 3 secs of 0 SD uptime are done,it will be up again, and if popped on cooldown, this cycle will continue endlessly (ending SD duration at 3 secs left on the CD).
However, to reach this stage, SD was active 100% for 51 full seconds. This of course is assuming best case scenario, where it is popped precisely whenever it comes off cooldown AND there is available Rage pooled in order to use it. But, to reach 51 full seconds of SD uptime, the charges must be pooled. This is where off-tanking comes in. The 1 minute mark (more or less) is pretty much ideal for tank switches - Durumu, Ji-Kun, Primordius (to an extent) Iron Quon, Jinrok, and more have tank switches that allow you to be off-tanking (not tanking the boss, that is) for more than 30 secs. You only need 27 seconds to recharge all 3 stacks of SD, and that's IF the switch happened as SOON as your last charge went on CD.
So, bottom line? When talking about bosses with tank switches, SD should have roughly 85%+ uptime (and that's being generous, but I'm assuming Human Error to be relatively mediocre towards high, here) when the boss is being tanked! When the boss is not being tanked, SD uptime does not matter - of course add-tanking is a different issue, but we're talking SD uptime where it matters, when tanking the boss that is.
Thoughts? My flow of logic wrong in any way? Discuss