Generally, it is OK to go by the roadway, but you'd better pay attention to Lhasa weather(https://www.travelchinaguide.com/climate/lhasa.htm) foarecast. The road may be closed if the weather is bad. Both bus and jeep are availability.
What do you mean of the time duration? From Zhangmu to Kathmandu, or the procedure time of the customs check at the border?

thanks for replying..i mean the duration from zhang mu to lhasa ..

it takes about 2 days, and you have to stay one night at Shigatse(https://www.travelchinaguide.com/cityguides/tibet/shigatse/).

hey mr ram...i am also planning to go back to nepal via lhasa along with a friend ..so if its possible we can make it to nepal together ...i heard its easier to rent a vehicle frm lhasa to jhangmu if we have a group ....hit me back ...