Depends on the complexity of the problem. But with only 1-2 mloc, problems usually aren't to complicated. If the problem is both serious and affects only a small part of the software, patches can be issued in 10-60 minutes after reporting the problem. More often though, it can take 1 or a couple of days before we fix a problem.
The key to fast response it the ability to deliver small patches that don't take a lot of effort to install. Our patches are installed on a central server, and then automatically installed on each of the clients whenever the client starts a program.