How to throttle requests in a Web Api?

ThrottleRequestWebApi.cs

[Route("api/dothis/{id}")]
[AcceptVerbs("POST")]
[Throttle(Name = "ApiThrottle", Message = "You must wait {n} seconds before accessing this url again.", Seconds = 5)]
[Authorize]
public HttpResponseMessage DoThis(int id) 
{
  // do something
}

public class ThrottleAttribute : ActionFilterAttribute
{
    /// <summary>
    /// A unique name for this Throttle.
    /// </summary>
    /// <remarks>
    /// We'll be inserting a Cache record based on this name and client IP, e.g. "Name-192.168.0.1"
    /// </remarks>
    public string Name { get; set; }

    /// <summary>
    /// The number of seconds clients must wait before executing this decorated route again.
    /// </summary>
    public int Seconds { get; set; }

    /// <summary>
    /// A text message that will be sent to the client upon throttling.  You can include the token {n} to
    /// show this.Seconds in the message, e.g. "Wait {n} seconds before trying again".
    /// </summary>
    public string Message { get; set; }

    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var key = string.Concat(Name, "-", GetClientIp(actionContext.Request));
        var allowExecute = false;

        if (HttpRuntime.Cache[key] == null)
        {
            HttpRuntime.Cache.Add(key,
                true, // is this the smallest data we can have?
                null, // no dependencies
                DateTime.Now.AddSeconds(Seconds), // absolute expiration
                Cache.NoSlidingExpiration,
                CacheItemPriority.Low,
                null); // no callback

            allowExecute = true;
        }

        if (!allowExecute)
        {
            if (string.IsNullOrEmpty(Message))
            {
                Message = "You may only perform this action every {n} seconds.";
            }

            actionContext.Response = actionContext.Request.CreateResponse(
                HttpStatusCode.Conflict, 
                Message.Replace("{n}", Seconds.ToString())
            );
        }
    }
}