caching name server cache expiration
I have set up a caching name server with bind/named on my fedora box. I have a root.hints too. When i run dig "some url" I get a response in a couple of 100's msec. When dig the same url again it comes back in zero or 1-2 msec. All good so far, the server is remembering. The problem I have is that it seems to then forget about 10-15 minutes later, we are back to the 100's msec in response to dig requests to the same url's. This also occurs with my LAN hostnames too. Slow response, fast response on the second request, leave it for 10-15 minutes and slow again.
Is there a way to increase the time the cache is kept for, say a month or so?