Two things:
1) Assuming that
convert(nvarchar,datediff(mi,Login_Time,Logout_time))) + ' Mins '
is the column you want as the duration, just add "AS Duration" to the end of it:
Select LogId,UserName,Login_Time,Logout_Time,convert(nvarchar,datediff(mi,Login_Time,Logout_time))) + ' Mins ' from UserLogs
Becomes
Select LogId,UserName,Login_Time,Logout_Time,convert(nvarchar,datediff(mi,Login_Time,Logout_time))) + ' Mins ' AS Duration from UserLogs
2) Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead. Building your command that way allows me to connect to your site from anywhere in the world and delete your entire database in about twenty simple keystrokes.