I'm Kay. I kill video game bad guys with my GW2 guild Rainbow Pride. We are an LGBT guild on European servers who raid casually 2 hours 3 times/week.
You can tell how hard a boss was by how loud we scream when it dies. If I had been recording our first Gorseval kill, it would probably shatter your windows.
For most of our raid Vale Guardian was their very first experience in raiding at all, in any game, we killed it in around 14 hours. Gorseval clocked in the longest at around 2 months. It was painful, frustrating, stressful and at times a horrible experience. Sabetha was maybe 3 or 4 weeks, it's hard to recall since we did the eternal title and immediately moved on to Salvation Pass.
Sloth was one of the easier bosses, comparable to Vale Guard and took maybe a couple of weeks. Bandit Trio should not count as a boss and we downed it on our first night after 4-5 pulls. Matthias was a struggle and clocked in longer than any boss other than Gorseval, taking around 6 weeks.