src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> timeout(long timeout, TimeUnit timeUnit, Scheduler scheduler, ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return timeout0(timeout, timeUnit, other, scheduler);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> timeout(long timeout, TimeUnit timeUnit, Scheduler scheduler, ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return timeout0(timeout, timeUnit, other, scheduler);
    }
Returns an Observable that mirrors the source ObservableSource but applies a timeout policy for each emitted
item using a specified Scheduler. If the next item isn't emitted within the specified timeout duration
starting from its predecessor, the source ObservableSource is disposed and resulting ObservableSource
begins instead to mirror a fallback ObservableSource.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/timeout.2s.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param timeout
maximum duration between items before a timeout occurs
@param timeUnit
the unit of time that applies to the {@code timeout} argument
@param scheduler
the {@link Scheduler} to run the timeout timers on
@param other
the ObservableSource to use as the fallback in case of a timeout
@return the source ObservableSource modified so that it will switch to the fallback ObservableSource in case of a
timeout
@see <a href="http://reactivex.io/documentation/operators/timeout.html">ReactiveX operators documentation: Timeout</a>