when to unlatch??

[superfrizbee]

DD is 8 days old and seems to be BF well... in fact often she wont unlatch on her own at all! I let her go over 30 mins on one side last night, burped her and offered her the other, which she took, but nearly immediately seemed to be sucking lethargically and not much interested. i popped her off nearly straight away and she wasnt bothered just went to sleep. im nervous about taking her off though because i dont want to affect my supply, but i dont know if she is even feeding at that point. I cant even hear/see when she is swallowing to make a judgement on that. is this normal? I dont know what to do!