There is no rule for that if one has valid muliple entry visa. However, this is generally related to mentality of the immigration officer (IO). The purpose of B2 visa is that someone come here for pleasure and return to Nepal. Coming back to US too soon will give th IO impression that there is no intention of staying in Nepal. There has been cases where the immigration officer granted just a month stay for second time just because the gap was very less. However, this varies from one immigration officer to other since it is their sole decision. Generally it is better to maintain a gap of at least 6 months or more to be on safe side.