Length of an interrupt
When the CPU handles an interrupt, it stops executing the main program. On small, simple microprocessors that lack interrupt nesting, it will also not serve other interrupts.

The interrupt service routine can execute for as long as your main program can tolerate it. You can try to estimate the number of cycles required if this really matter to you, but your code seems quite short to me.

The size of the code is not an issue ether. MicroP uses vectors to jump to the service routine code, so it can be as large as needed.