I've struggled with this for awhile. I've always wanted it to be at ten minutes, but occasionally I see it reverting back to 30 seconds, particularly on a soft reboot. The ONLY other thing I can think of (and this is pure speculation) is the app I'm using called Juice Defender, which has a screen timeout setting built in which I don't entirely understand, which could be re-setting the timeout interval to save battery or due to the time of day.
Once again, it's the only thing I can think of, and I've had this phone since Apr 29th. Anyone else having this issue?