As I've been running retrospectives, I've noticed something interesting around how much time we spend on them.
I've been on teams that decided to run retrospectives every 3 weeks. They picked a list of oversized changes, which didn't get implemented, so people stopped seeing the retros as valuable. If there was a scheduling conflict, a retro would get skipped, which just made the problem worse.
On my current team, we do retrospectives every week. The first one took 90 minutes, and triggered a lot of comments complaining about how long it took. I knew that if I wanted to get people to actually show up to weekly retros, I would have to limit the size to the more socially acceptable 60 minutes.
One tool I used was to ask each person to arrive with exactly one item (rant or rave) that they want to give attention to. That helped a lot with the time spent, without hurting value too much (although after a few iterations, we got more efficient from practice and people asked to go back to multiple items.)
I actually think we're optimizing the wrong way, though: if 90 minutes / week isn't worth it, then instead of reducing the cost, I'd rather increase the value. I prefer effectiveness over efficiency. There's a lot of latent potential upside here, focusing on efficiency optimizes for the status quo. I'd like to accelerate our improvements by having retros even more often (daily?!?!), but I am sure that idea would meet heavy resistance (and undermine my credibility.)
I've noticed an odd phenomenon that suggests that optimizing for time is not the right choice: at the end of each formal retrospective, when the meeting is officially done, about 1/2 the group sticks around and continues to discuss how we work. This sometimes goes on for another 90 minutes. Something similar happens at lunch. So clearly, there is a real need for more of this introspection. I just think that the formality of the scheduled retro is something people can't tolerate for more than 60 minutes / week.
It's strange that I hear the comments about "wasting too much time on retros", while people's actual actions show that they really are interested in devoting their time in this space.