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Sounds pretty normal to me.
The ad says "8 week battery life (30 mins/day, wireless off, light at 10)". So let's do the math: that actually means 8*7*0.5 hours non-stop, meaning about 28 hours. Your 8-9 hours would be ~1/3 of that, so arithmetically, you should have ~60-70% left according to the ad. Other factors are light intensity setting and few uncertainties/"measurement errors" here and there.
IMO, there's nothing to worry about.
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