What I saw - first they have to reach 75% of nodes for a full difficulty period.
Then they will maintain 75% of nodes for two additional difficulty periods while getting the word out that the fork is coming.
Then the fork can happen.

So it will take at least 6 weeks from when they hit 75% - and they are only in 30% range now - http://nodecounter.com/#bitcoin_classic_blocks - only about (just under) half the mining hash power needed.

They don't have to stick to that plan but won't fork w/ less than 51% and I doubt they will do it with less than 75%.