hi,
The standard 8051 takes 12 clocks for each instruction cycle
1) not for each one. There are instructions which take 24 and 48 cycles.
2) It may take much more clock cycles in real application if one or more interrupts are enabled. If be 100% correct, an instruction still takes defined time but it is not truth for time sum of two or more instructions (here is 2+2 != 4 :))

cu