How Long will it take to lose a stone if i use a tread mill 30 mins aday 3 days a week. and eating under the DRA calories

You should aim to lose one-two pounds a week in order to do it healthily and keep the weight off in the long term.
So between 7-14 weeks

Ideally you should aim to lose between a pound to two pounds a week, at this rate it should take at a minimum 7 weeks.

About two seconds if you drop it------- 14 lbs at two pounds a week provided correct diet and reasonable work out , so six weeks approximately

depends on your metabolism.