measuring interrupt interval with app_timer

I have an interrupt coming in every 50-150 microsecs and I want to measure these intervals as precisely as possible.

I tried this with SDK12.2:

APP_TIMER_INIT(0, 5, false);

then in the (GPIOTE) ISR:

uint32_t this_time=app_timer_cnt_get();

I always get zero back;

Presumably I need to initialise it in a different way?

Thx for any suggestion.