RateLimit.cs

// Only let input Observable fire every 'n' seconds at most
// but unlike Throttle, items fire immediately when they aren't
// rate-limited.
public IObservable<T> RateLimit<T>(this IObservable<T> This, TimeSpan interval, IScheduler scheduler)
{
    var slot = default(IObservable<Unit>);
    var input = This.Publish().RefCount();

    return input.Window(input, _ => {
            if (slot != null) return slot;
            slot = Observable.Return(Unit.Default);

            return Observable.Return(Unit.Default)
                .Delay(interval, sched)
                .Finally(() => slot = null);
            })
        .Select(x => x.Take(1))
        .Merge();
}

RateLimitTest.cs

[Test]
public void RateLimitingTest()
{
    (new TestScheduler()).With(sched => {
        var input = sched.CreateHotObservable(
            sched.OnNextAt(100, Unit.Default), // Yes
            sched.OnNextAt(200, Unit.Default),
            sched.OnNextAt(300, Unit.Default), 
            sched.OnNextAt(1000, Unit.Default),
            sched.OnNextAt(1200, Unit.Default), // >1100, Yes
            sched.OnNextAt(2100, Unit.Default), // Not >1200+1000, No
            sched.OnNextAt(2300, Unit.Default) // > 2200, Yes
        );

        var slot = default(IObservable<Unit>);
        var output = input.RateLimit(TimeSpan.FromMilliseconds(1000), sched)
            .Timestamp(sched).CreateCollection();

        sched.AdvanceToMs(50);
        Assert.AreEqual(0, output.Count);

        sched.AdvanceToMs(150);
        Assert.AreEqual(1, output.Count);

        sched.AdvanceToMs(1000);
        Assert.AreEqual(1, output.Count);

        sched.AdvanceToMs(10 * 1000);
        Assert.AreEqual(3, output.Count);
    });
}