Just use a timeout and a flag. If the timer runs out, set the flag and reject the promise. Drop the result once it arrives (too late). If the result is available early enough, it can clear the timer and resolve the promise.

(async () => {
  // some other code

  let result;
  try {
    result = await new Promise(async (res, rej) => {
      // the flag:
      let handled = false;

      // set timer
      const timer = setTimeout(() => {
        // the timer wasn't cleared, so reject the promise
        rej(new Error('timeout'));
        // and set the flag for the late result
        handled = true;
      }, 30000); // timeout in ms

      const result = await veryLongAsyncFunction();

      // resolve the promise, if it wasn't rejected before
      if (!handled) res(result);
      // and clear the timer
      clearTimeout(timer);
    });
  }
  catch(err) {
    // handle error, for example timeout
  }

  if (result) {
    // handle result
  }

  // more code
})();