Array enter (enter using Ctrl-Shift-Enter)

=SUM(IFERROR(TIMEVALUE(MID(G8,1+(ROW($A$1:$A$12)-1)*12+6,5)),0)-IFERROR(TIMEVALUE(MID(G8,1+(ROW($A$1:$A$12)-1)*12,5)),0))

If you want hours (for your multiplication) use:

=SUM(IFERROR(TIMEVALUE(MID(G8,1+(ROW($A$1:$A$12)-1)*12+6,5)),0)-IFERROR(TIMEVALUE(MID(G8,1+(ROW($A$1:$A$12)-1)*12,5)),0))*24

also array-entered.

If you could have more than 12 rows of time differences in one cell, change the $A$12 to $A$-something larger.