I think it's pretty clear that chaining 20sec APs is impossible, but perhaps a 30sec AP might be viable (which lines up nicely with Evocation's cooldown, 1min or 2min CD trinkets).When I say "a 30sec AP", I mean an AP with cooldown of 27sec, but cast every 30sec.
In order to do that, we'd need 9 stacks of Stolen Time; we would then have to cast 9 ABs in 15 sec. Let's say you'll keep a 4ABAM rotation. Let's also assume you'll be on 1268 haste - with the extra 500 from 10 stacks of Stolen Time, you'll be at the soft cap. This would give us a cast time of:
1.68sec for Arcane Blast 0 stacks
1.60sec for Arcane Blast with 1 stack
1.51sec for Arcane Blast with 2 stacks
1.43sec for Arcane Blast with 3 stacks
1.35sec for Arcane Blast with 4 stacks
So, casting 4ABs would take us 6.22sec and let's say we interrupt AM as soon as we can (that is, we channel it solely for our GCD duration of 1.26sec). Then 4ABAM takes 7.48sec. To get 9 stacks of Stolen Time we'd need to do (in this scenario) 4ABAM4ABAMAB. This would give us a total time of 16.64sec, falling short of our gal of 15 sec.
Notice, however, that while we're casting those ABs we are gaining haste from Stolen Time stacks, so the time it takes for us to get there is actually less than that. I suspect that due to client-server delay, an AB you cast will not benefit from the stack you gained from the immediately preceding AB, but we'll have to test that. Taking all of that into account, I anticipate we might need a little more haste to reach 30sec APs; I will try and model that later.
Please correct me if any of that math is wrong. To be honest, though, I'm more worried about how our mana usage will go around with this set's bonus and with keeping APs on the run. Perhaps we'll need a less mana intense rotation for that, say 3ABAM, and then the time it takes to get to our desired number of stacks change.
I'm very excited about this set's bonus.