There are 2 routes that can be taken to reach Leh via road.
1. From Manali to Leh
2. From Srinagar to Leh
Apart from these 2, a 3rd and longer route can be to start from Shimla, cross Spiti Valley and merge into the Manali leh highway near Kokhsar.
Alternatively, one can also start from Chamba / Dalhousie, continue towards Killer, cross Sach pass and continue on the road that merges into the Manali Leh highway near Keylong.
The 3rd and 4th routes are just an extension to the Manali Leh highway because eventually these will lead you to the same road on the other side of Rohtang Pass that goes from Manali to Leh.
Manali to Leh can be done in 2 days but that becomes a little hectic because you will be rushing through all the way with very little time to enjoy the natural beauty all around you, which in turn beats the very purpose of the journey. Recommended time to complete the journey is a minimum of 3 days and 4 if you have ample time in hand. This will give you ample opportunity to immerse yourself in the scenic beauty of Ladakh.
Same applies for Srinagar to Leh route, only difference is that this journey can comfortably be completed in 2 days, without rushing things through as opposed to Manali to Leh route. However if time allows, you can complete the journey in 3 days to make it more pleasant and enjoyable.
Other 2 routes, as i mentioned earlier, are just added extension to Manali Leh highway. Any amount of time you take to reach the Manali Leh highway near Kokhsar or Keylong will be an addition to the 3 days that you will need to reach Leh from Manali. Journey from Shimla to Rohtang via Spiti valley can take 4 – 7 days depending upon what all areas you are planning to explore. Reaching Keylong from Dalhousie or Chamba will take 3 days, if traveled at a convenient pace and 2 if rushed through.
So the route to Leh from Shimla via Spiti Valley can take 7-10 days and from Dalhousie to Leh via Sach pass can take 5-6 days.
Remember, the real fun and pleasure is the journey itself and not in reaching the destination. Awesome and breath-catching scenery and not-even-thought-of surprises along the highway should be savored leisurely.
Click on the links below to read about the above mentioned routes in details.
I hope this information was helpful. If you have any further questions, please feel free to comment and ask and I would be glad to answer.