According to a quick Google search. Xp/h varies from 400-720k/h.
It's a very expensive method though. I assume from that, 35-70 cooking would take between 1 1/2 - 2 hours using this method, assuming that you don't go as fast as possible for the whole two hours.
Thanks for the replies, currently got from 35-55 cooking so far with them, I don't mind about the price but grapes are struggling to buy on the GE... :/