September especially first two weeks will be the best time to visit Leh and Regarding no of days if you are planning to go via Road from Srinagar and Return via Manali it will take minimum 10 to 11 days. There is direct flight to Leh from Delhi and if you wish to see places like Diskit, Hunder, Pang
Leh Ladakh is always amazing place to visit throughout the year but some of the roads are closed this time so i will suggest you to cross check first the places you want to visit there are accessible via road or not. thanks.