Heres a post from a guy working at CERN right now (from another forum)
"The current estimate for 'safe levels' is about a week after shutoff, but that'll increase the longer you run the collider for. Even after a week, we'll be limited in how long any one person can spend down there. If you went down immediately, you're very unlikely to do yourself any serious harm, but you'd probably get a hike in your life insurance premiums. CERN abides by the exact same occupational health requirements as those imposed on anyone else as to safe levels of radiation, and the current max dosage limits are very low with respect to long-term effects.
This is all based on current estimates mind you- we monitor rad levels in the caverns, and the safety officer will only give the go ahead once they drop below those limits. If we've misestimated, we could be scratching our arses for a few weeks!"