Hi Isabeau, IMHO... I think it's most fair to charge per knitted yard. Charging per hour opens up the possibility for biased fees in that quick knitters could "undercharge" and slow knitters could "overcharge". In addition to cost of yarn, your per yard rate could be between $0.10 and $0.25:
$.10 - really easy garter with no shaping
$.25 - complicated cables or lace work
By no means am I an expert, but I did some internet surfing about the topic a couple years ago - which lead me to the per yard school of thought. (I might have even posted about it on KH; I can't remember
) Anyhoo ... it's all very subjective to each knitter, but I hope this helps in some way.
Laptop Knitter - thanks for the referral!! I appreciate it very much.