I would use an array formula (ctrl+shift +enter)

A=Product
B=Start time
C=end time

=SUM(IF(A1:A10="driwall",IF(C1:C10<B1:B10,(1+C1:C10)-B1:B10,C1:C10-B1:B10)))

=SUM(IF(A1:A3="hardwall",IF(C1:C3<B1:B3,(1+C1:C3)-B1:B3,C1:C3-B1:B3)))


let me know if this works