Consultation fee?

Hi All! I have just been contacted by a jewelry company in NYC who
would like to have me be a design consultant for them. They said to
think about what kind of fee I would like to receive for my
services. I would also be making a certain percentage as a
commission on the designs of mine that sell. I have never done any
consultation work and have no idea what to ask for. So far my time
spent will be the initial meeting today, a conference phone call
tomorrow, and going back into NYC to meet with them again on Wed.
This is in addition to any time I spend working on ideas for their
line. Do I keep track and charge on an hourly basis? Weekly basis?
Any thoughts on this would be much appreciated!