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In my experience there's only so much you can do about light pollution. It'll always create a background level in your images, which will often be an oddly shaped gradient and potentially a bugbear to remove. I suspect it's easier to remove in LRGB than single-shot colour, but I have no experience to back that up.
I've never done any narrow band imaging but it would seem that the narrower the band, the less likely light pollution will strongly affect your images (particularly in H-alpha - which doesn't match common artificial wavelength).
To determine how long you can image just image an emptyish patch of sky at different exposure lengths and check the histograms of these to see how much back contribution there will be to your images. That's the amount you'll be wanting to cut off your actual images.
I hope that's some help.
-Cam
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