Hours, though if the admin can't do everything in one go, it might be out for a day. Both drives are displaying errors. To quote him. "Two of the same drives from the same batch, in a RAID config getting the same usage". The good news is this doesn't sound like a catastrophic sudden onset of bad sectors. Or maybe it isn't on the so called "good" drive". I'm trying not to be a huge pest. But either way, the drive in the best condition is NOT a candidate for being in the new install. So both drives will be replaced and all the VMs, including mine, will be saved as disk images and then restored. He'll be doing this when he gets back from work most likely and will probably take the better part of the evening.
I personally have backups of my mySQL tables so in the worst case I can eventually get stuff back together if somehow everything fails, but our Admin is awesome and has been doing co-lo stuff for years. I think part of this and the warning about Dataloss isn't because it's necessarily likely, but he treats it with an admin's expected level of severity and professionalism.
Edit: Little more info. Both drives are exhibiting bad sectors but the situation isn't dire like I said. But it is a sign that the drives have to be replaced sooner than later and that'll happen tonight. No known data lose and hopefully nothing will go wrong during the transfer.