How much weight is reasonable to lose?

If I have fruit banana and cherries & orange and a little yoghurt (55. Calories ) a pot for my breakfast with a coffee skimmed milk. For dinner a slimfast shake then for tea food like a jacket potato fish rice chicken. All bland as I'm fussy eater! Is it reasonable to lose a stone in 5 weeks? Do I exercise alongside it have zumba that I'm doing 3 times a week for 30 min on the kinect plus extra dog walking!