Answer for https://stackoverflow.com/posts/55309283/edit

index.js

const { rxObserver } = require('api/v0.3');
const { Subject, interval, merge } = require('rxjs');
const { timeInterval, filter, distinctUntilChanged, map , pairwise } = require('rxjs/operators');


const START_EVENT = 'RUN';
const END_EVENT = 'END';
const events$ = new Subject();

const result$ = 
  events$
    .pipe(
      timeInterval(),
      pairwise(),
      filter(([first, second]) =>
        first.value === START_EVENT
        && second.value === END_EVENT
      ),
      map(([, second]) => second.interval)
    );


events$.subscribe(rxObserver());
result$.subscribe(rxObserver());


// push events
setTimeout(()=>{
  events$.next(START_EVENT);
}, 100);

setTimeout(()=>{
  events$.next(START_EVENT);
}, 200);

setTimeout(()=>{
  events$.next(END_EVENT);
}, 550);

Run at https://observable-playground.github.io/gist/73877d92ddc6ca8e4a53af1e8effb688