Multiple Response AJAX request

You can implement this using 2 ajax calls, one to run the process and a second call to periodically poll the progress:

On the server side:

public class ProgressInfo
{
    public int Percent {get;set;}
    public bool Done {get;set;}
}

public JsonResult DoCalculation(string id)
{
    ProgressInfo progress = new ProgressInfo();
    if(!string.IsNullOrEmpty(id))
    {
        Session[id] = progress;
    }

    //periodicly update progress
    progress.Percent++;
}

public JsonResult GetProgress(string id)
{
    ProgressInfo progress;
    if(string.IsNullOrEmpty(id)
        || (progress = Session[id] as ProgressInfo) == null)
    {
        return Json(new {
            success = false
        });
    }
    if(progress.done)
    {
        Session.Remove(id);
    }
    return Json(new {
        success = true,
        done = progress.done,
        percent = progress.Percent
    });
}

On the client side:

var progressID = Math.random();

function doCalculation() {
    $.post('<%=Url.Action("DoCalcluation")%>/' + progressID);
    setTimeout(pollProgress, 1000);
}

function pollProgress() {
    $.post('<%=Url.Action("GetProgress")%>/' + progressID, function(response){
        if(!response.success) {
            alert('Cannot find progress');
            return;
        }
            if(response.done) {
                alert('Done!');
            } else {
            alert('Progress at ' + response.precent + '%');
            setTimeout(pollProgress, 1000 /*1 second*/);
            }
    }, 'json');
}