Sprint planning - how long is too long?

I posted one of my articles, Sprint planning meeting introductory script, on reddit to try and stir up some conversations, and the following comment by /u/AgileBandit stuck out to me:

3 hours? Why are your Sprint planning sessions taking so long?! Your team would benefit from more frequent backlog grooming sessions to reduce the need for lengthy Sprint planning session – 45 minute sprint planning sessions should be your goal.

I understand what he’s getting at here. If the team fully understands the work before the sprint planning session, then it’s just selecting the items from the top of the backlog, having a quick discussion to remind people about what the work entails, and you’re off to the races. The main reason that would be possible would be spending the potential 2.25 hours throughout the sprint in backlog grooming sessions while I don’t like spending more than an hour (more if we just changed the area of work to which we’ll be focusing).

I don’t agree with his approach for a few reasons, namely to avoid wasted effort, share knowledge throughout the team, and leverage experience.

The timing of focusing on this issue earlier introduces a risk of wasted effort by tasking out a story that ends up being de-prioritized before you reach the sprint planning meeting. Any effort you put in past understanding whether you knew all of the information that was required to build up proper acceptance criteria would be thrown out the window.

Backlog grooming sessions don’t involve the whole team, so having the discussions of how you plan to approach the issue with a subset of people limit the full understanding only to those who were present. This can introduce some risk by limiting the full understanding to a subset of the team, if they’re tied up working on other stories.

45 minutes doesn’t seem like it would give you enough time to amply task out your stories. In addition to the point above about timing for lost effort focusing on the issue, there is also the risk of not using your latest experience to factor in to how you want to task out your stories and learn from previous mistakes and successes.

From all the advice I’ve been reading online, It mentions that an 8 hour sprint planning meeting for a month-long sprint is appropriate, so I don’t feel that a 3 hour meeting for a 2 week sprint is anything out of the ordinary. Because of this and the fact that I’m a huge fan of reducing risk as much as possible, I won’t expect my sprint planning meetings to move close to 45 minutes.