The correct way to calculate MIPs of a function on a microcontroller
As you may know, MIPS means Mega Instructions Per Second.
So the formula would be

CHIP_MIPS = number_of_mega_instructions / number_of_seconds.

If your chip is running at 25 MHz, it means that it performs 25 mega instructions per seconds, so knowing the number of seconds and the CHIP_MIPS, the formula would be

number_of_seconds * CHIP_MIPS = number_of_mega_instructions

So fun needs 5 Mega Intructions