Does anyone know in cycles how long it takes for the Z80 to start executing an interrupt service routine after acknowledging the interrupt?

it depends on the interrupt mode of processor.

in mode 0 - it takes 13 or 19 clock cycles. 13 cycles if interrupting device placed 'RST x' on data bus. Otherwise if 'CALL x' is on the data bus, it takes 19 cycles.

in mode 1 - it takes 13 clock cycles.

in mode 2 - it takes 19 clock cycles.

in mode 0 - it takes 13 or 19 clock cycles. 13 cycles if interrupting device placed 'RST x' on data bus. Otherwise if 'CALL x' is on the data bus, it takes 19 cycles.

in mode 1 - it takes 13 clock cycles.

in mode 2 - it takes 19 clock cycles.

oh, and for NMI it takes 11 cycles.

Thanks a lot arafel :D