I think most babies need an average of about 750mls a day. You divide this between how many feeds a day. Mine usually had about 800mls but sometimes as low as 700mls. I've heard that some BF babies hold out on drinking a lot until they see their mum again.

In terms of defrosting I agree with KMN, just defrost under the tap. Microwaves destroy some of the properties in BM.

The website that Dragon Cookie posted is a great link and gave me so much information about using, handling and storing BM. It includes a calculator to work out how much a bub needs per feed.

Also slow teats at this age, but at any age if you want to continue on the breast.