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Alistair, my experience is that they only have to be a few degrees above ambient to work for most applications. I test that they are on by pressing my lip against them. You will feel that they are warmer than the surrounds.
But the best way to use them is to have them on from the beginning, rather than turning them on after everything is dewed-up. Start off on low and crank them up as the dew starts to show. So get a big battery or run off a mains-powered supply.
Eyepieces and finderscopes you can crank up and down no problem. I'm guessing the front lens of a small refractor shouldn't be a great problem. But be a bit more cautious of the corrector plate on a bigger SCT. I find corrector on my 8" SCT doesn't like rapid changes in temperature. Before I had a heater strap, I hit it with a hair-dryer one evening while someone was observing - nauseous results for the observer!
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