Ever since they cut the duration of Demolish to 18s I've had occasional trouble keeping it up 100% of the time as well. I haven't done any serious number-crunching on it but I've noticed that depending on where in the rotation RoF is activated Demolish sometimes doesn't fall off/have to clip, so I guess there's an optimal place to activate it rather than the instant it comes off cooldown, unless simply letting it fall off for a second or two to activate RoF immediately would be a better DPS gain.

Of course, differing SS levels can heavily impact all of this. No clue on what would be optimal, I just know i'm sitting somewhere 'round 1100