I don't know, I'm afraid. According to the Wikipedia page on the LHC, though, injection takes "several minutes", acceleration after that takes "20 minutes", and

I don't know, I'm afraid. According to the Wikipedia page on the LHC, though, injection takes "several minutes", acceleration after that takes "20 minutes", and then the beams circulate for 5-24 hours while the detectors watch for products of collisions. (Beams plural because the LHC has two beams circulating in opposite directions.) I would expect dumping to be very quick.