Usually you'd have an internal timer running free, and sample its count register in the interrupt. Subtract from the value read on the last interrupt, and adjust for overflow (since the timer's count is modulo MAX_COUNT).
The timer reset interrupt might increment a global counter, to make this information available.
Warning: be careful with interrupts on switch contacts. Add some kind of debounce function. Whether analog (RC filter, hysteresis) or digital (e.g., interrupt disables itself and sets a timer to re-enable it, thus making a holdoff action), it should prevent the pin from triggering too many interrupts.
It is very much a thing, that poorly designed systems have locked up from ambient noise triggering pin-change interrupts. Just a basic precaution.
Tim