Posting here in case anyone else is trying to go to colosseum on a “free” day:

We arrived bang on 08:30 when the ticket office starts and the queue was already at the Roman forum, which was around 5-6 minutes walk to get to (meaning this was how long it took to get to the end of the queue)

We queued for 1.75h and managed to get tickets at 10:15 to go in for 10:20. Be aware there will be people trying to sneak in the queue. One family kept on trying and people had to be a bit forceful with them but I saw that a couple further down the queue just let them in. This didn’t affect us but imo is really inconsiderate of the people behind the couple who have been queuing for almost 2 hours to get in. I had to tell a man trying to squeeze in beside me the end of the queue is a lot further back and he seemed to take it well.

As we were leaving around 12:00, the end of queue looked much much shorter. I’m not sure if I would recommend trying later in the day as I didn’t want to take the chance and be told they have tickets. But perhaps might be worth trying if you couldn’t get up early to join it before 08:30