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I guess it depends on how much you stop the lens down.
At 10-22mm, you're taking in enormous swathes of sky; equivalent to about 17-40mm in a full frame system.
You'll probably be looking to shoot at between f/4 to f/5.6 to reduce chromatic aberrations.
I used to shoot for 10-minutes at f/4 at ISO-800 on the 40D.
Flat frames are an absolute necessity when shooting widefields -- the outer portions of the image will receive somewhere up to 20% less light than in the image centre. Vignetting is wonderful for terrestrial photography (framing people/landscapes, etc.,) but it is detrimental to astrophotography.
Taking flat frames is easy. Set your camera to ISO-100 (its lowest native setting), point your camera at an evenly illuminated flat object (wall) and take exposures that give you a histogram on your LCD that peaks about 2/3rds the way across the X-axis. The exposure length will be dictated by your light source.
One thing to remember is to NOT remove the lens from the camera until after you've done your flat frames. That is, once you've captured your light frames, do not rotate the lens, or change focus or focal length -- once you do that, you will shift the artefacts (dust bunnies, streaks, random particulates) to a different location/focal length, and will induce further aberrations in the finally calibrated image.
When I used to shoot with a camera and lens, I would carefully remove the camera from the mount, plonk it down on my laptop's keyboard, open a blank Photoshop document with a white background, and point the camera at it. Then, just used the metering in the camera to give me a proper exposure in aperture priority mode, and then rattled off 16-25 RAW flat light frames. Once that was done, I'd put the cap on the lens, and take flat dark frames -- same exposure duration and ISO as the flat light frames. Median combine the flat dark frames into a master flat master dark, and, subtract that from each flat light frame. Then, median combine the flat light frames into a master flat field.
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