ranii, please visit usembassy website of nepal.  you dont need all those documents what these indian forums says.  believe me, its lot easier than people think from here.  4 weeks should be enough if you are going in january.  remember, by that time most of the students or parents who are coming here for their kid's graduation would have already applied for visa. 

what i had during my interview:

H1b approval notice ( I 979)

transcript

offer letter

resume

passport

photograph

you will submit your application in Nabil bank to get the visa interview date and time..