It would be effective immediately just like the -tohit debuff is effective immediately - however it would actually have a real effect on the mobs, because stunned mobs can't use tohit checks. Stunned mobs getting immobilized won't wander around dispersing however.
Like I said, the more pieces of the set you get, the less useful the proc becomes, so the set is actually fighting against itself in the same power, which is kinda bonkers. I guess it's like how putting damage procs in sleep powers used to be before the Deep Sleep mechanics were added in the very last patch, only this is worse, because at least the damage would have a real effect.
That mob that Snarky stunned that can't use any tohit checks now has their tohit debuffed - and once mag levels were introduced into the conversation, I asked which mobs the proc was for that were resisting his mag 4 disorients so I could understand his goals - wouldn't a good chunk of bosses get stunned at mag 4? Once he answered it contained information that didn't seem to be a reply to my question, so I confirmed his statements and asked again politely.