Hi Sunil, The best time to visit Ladakh would be somewhere between mid June to mid July. During this period, it is not too cold since summers have just started and you can witness both the barren landscapes and snow covered peaks.
Any time before that would be very cold. Also most passes will be closed as roads are covered with snow and dangerous to travel on. Visiting Ladakh in August-September period is also great since its a safe period to travel, however you will not be able to see much snow covered peaks during this time.
I think a trip of about 10-12 days should be enough to cover all the major attractions in Leh & Ladakh.