Length is Mb - they're one and the same thing.
This is why CDRs are refered to as 80 minutes long and 700Mb capacity for example. 74 minute CDs have a lower capacity (640Mb I think?).
So anyways, if you have a 700Mb wav thats at CDA specifications (41,000Hz, 16 bit, 2 channels) it's 80 minutes long.