I found that actively using my iphone for 30 minutes will take out 5-7% of the battery. Is it normal?
Yes, this is normal. If we assume that actively using your iPhone 4 for 30 minutes consumes 5% of its battery life, then you can use it actively for 10 hours. The iPhone 4 supports up to 10 hours of uninterrupted video playback OR up to 7 hours of talk time on 3G OR up to 6 hours of Internet use on 3G, for example. Your experience with your iPhone 4 sounds reasonable.