A userscript that displays how long it took for a StackOverflow question to be closed as a duplicate.

SO_dupehammer_stopwatch.user.js

// ==UserScript==
// @name         StackOverflow hammer stopwatch
// @description  Displays how long it took for a question to be closed as duplicate
// @version      1.2
// @author       Paul Pinterits
// @include      *://*.stackexchange.com/questions/*
// @include      *://meta.serverfault.com/questions/*
// @include      *://meta.stackoverflow.com/questions/*
// @include      *://meta.superuser.com/questions/*
// @include      *://serverfault.com/questions/*
// @include      *://stackoverflow.com/questions/*
// @include      *://superuser.com/questions/*
// @exclude      *://*/questions/tagged/*
// @exclude      *://*/questions/ask
// @grant        none
// @updateURL    https://gist.github.com/Aran-Fey/c6f87944795d579f8807d2ba6741a150/raw/SO_dupehammer_stopwatch.user.js
// @downloadURL  https://gist.github.com/Aran-Fey/c6f87944795d579f8807d2ba6741a150/raw/SO_dupehammer_stopwatch.user.js
// ==/UserScript==


function extract_time(node){
    var time_node = node.querySelector('.relativetime');
    var time = time_node.title;
    return Date.parse(time);
}

function format_time(time){
    var sec_num = Math.floor(time/1000);
    var hours   = Math.floor(sec_num / 3600);
    var minutes = Math.floor((sec_num - (hours * 3600)) / 60);
    var seconds = sec_num - (hours * 3600) - (minutes * 60);
    
    var result = '';
    if (hours > 0)   result += hours + 'h ';
    if (minutes > 0) result += minutes + ' min ';
    if (seconds > 0) result += seconds + ' sec ';
    return result.substr(0, result.length-1);
}


var question = document.getElementById('question');
var ask_time = extract_time(question.querySelector('.owner'));
var seen = [];


function show_time(){
    for (var status_box of question.querySelectorAll('div.question-status')){
        if (status_box === null)
            continue;
        if (!status_box.textContent.includes('marked as duplicate'))
            continue;
        if (seen.includes(status_box))
            continue;
        seen.push(status_box);
        
        var close_time = extract_time(status_box);
        var duration = close_time - ask_time;
        
        var elem = document.createElement('span');
        elem.classList.add('relativetime');
        elem.textContent = '(Time: '+format_time(duration)+')';
        var parent = status_box.querySelector('h2');
        parent.appendChild(elem);
    }
}

show_time();

function on_dom_mutation(mutations, observer){
    for (var mutation of mutations){
        if (mutation.addedNodes.length > 0){
            show_time();
            return;
        }
    }
}
var observer_config = {childList: true,
                       subtree: true};
new MutationObserver(on_dom_mutation).observe(question, observer_config);