well, i'm not really sure about kefir fermentation, i thought it required some extra materials than a seed culture
in any case, lactobacillis acidophilus fermentation is so simple you almost can't fail to get it to work, the main tricky thing is keeping the temperature nice
generally overnight is enough for your basic yoghurt but if you want it to be very little lactose remaining you need to run it for 36 hours (IMO better to let it run longer because lactose makes you fat)