I'm wondering, how long will all the settings remain in the 990 if I remove power from it? I'd like to unplug it during lightning storms but I'm afraid it will lose the settings.

I don't think you'll lose anything by unplugging it overnight. In fact, I'd expect it to remember for much longer than that - my old Model 950 remembered all of its settings through a 10.5 day power outage about three years ago.