My brother taught his wife to sail up at Geraldton last November(very interesting) and believe it or not we were up there for the month and only got 4-5 good days so good for learning i guess but she was using anything from a 4.4-5.2 wave sail and he got her into the harness as soon as possible in the lighter days to start with and i must admit there was some frustration on bro's behalf

as she did'nt feel comfortable using it but as everything the more you do it the more comfortable it becomes,

so from what i've seen i would say when theres enough wind for her to leaning back slightly on the boom introduce the harness.
My sister inlaw now to my brothers dicussed she is now sailing around on his brand new Quatro 85ltr freestyle wave. A great effort i think