The units are ready 24/7, pilots are in flight gear, G-suits zipped up and planes ready (meaning internal INS lined up. This takes some time normally) ready to jump in and take-off within 5 minutes. Usually there's two types of scrambles: Alpha and Tango. Alpha being the real thing and Tango for training. I am not familiar with normal ops out of Portland, but back in the Cold War days our nearby US F-15's out of Soesterberg airbase in Holland conducted Tango scrambles quite regularly to keep pilots trained. Alpha scrambles were very rare except for this Russian MiG-23 without a pilot transiting the East-west DMZ in 1990. It is logistically expensive, but especially since 9/11 these units are ready. Not only in the US, but in Europe too (as well as other parts probably)