650 sks: 2.50245-((650-354)*0.0003776) or 2.3906804
708 sks: 2.50245-((708-354)*0.0003776) or 2.3687796
GCDs until an extra GCD: 2.3687796*(x+1) = 2.3906804*x
Solve for X: approx 108.
So in the time you get 108 attacks with 650 SKS, you could have gotten 109 attacks with 708 SKS. This occurs at 258.1934832 seconds into the fight, or 4:18. HOWEVER, note that every time a phase change occurs where you cannot attack (every A3S phase change for example), the 4:18 timer effectively resets and you've lost any progress you've made toward the extra GCD. Note that this is also true any time you delay a GCD. This means that if in phase 4 of A3S you have to walk away for Ferrofluid and the GCD isn't being used for a short amount of time, you have again reset your progress toward the extra GCD. This means you need 4:18 of unbroken attacking in phase 4 in order to see the benefit compared to someone running 650 SKS. Phase 4 is only long enough for one stretch of that, so you only gain one GCD in the entirety of phase 4.
Lets assume that the best case is that the extra GCD is a Full Thrust (360 potency) and we have 1079 STR, 76 WD, and 357 DET. That means that before buffs + crt, the FT will deal 1834.51132181 damage (according to the formula ((Potency/100)*(WD/25+1)*(STR/9)*(DET/7290+1)*BUFFS)-1).
Since we get approximately 395 total GCDs + oGCDs here, the effect of 39 DET assuming an average of 220 potency per hit is about 5.7 damage per hit, or 2251.73436306 total damage over the 395 GCDs. As far as DoTs vs Autoattacks, DET scales better on AAs than SKS does, and AAs are responsible for more total damage (about twice as much) as DoTs are. Since 58 SKS is not more than twice as much as 39 DET, even before the better scaling of DET, we can conclude calculating AAs and DoTs would only further shift the balance in favor of DET.
Anyway, all this is to say that as bad as DET is compared to STR and WD, SKS is basically trash in practical situations due to phase transitions.
Edit: This is incorrect - further math has proven that skill speed is actually good and I eat my words.