A minimum of 5 days is good for covering all the important places in Ladakh.
The itinerary for the trip:
Day 1 Reach Leh – Get Acclimatize; take a visit to shanti stupa
Day 2 Roam around Leh , visit Leh palace or instead go to magnetic hill and drass war memorial
Day 3 Visit Nubra Valley
Day 4 Back To leh relax and do some sightseeing (or go to Pangong Tso)
Day 5 – visit Hemis Monasteries and come back to Leh