For a new product launch that is under development, when is the right time to start planning and meeting with the different channels?

This will depend on the Go-To-Market strategy and which channels you plan to leverage. In my experience, once the roadmap is defined and you have developed your v0 GTM, that is a good time to start meeting with channel owners to seek feedback on the proposed plan and align around the channel strategy. Once your plan is vetted, then I like to partner with a program manager to develop a work back plan that will inform the right time to engage each channel owner. If your org doesn't have a program management team, you can work with the channel managers directly to understand their timelines.

When you launch as quickly and regularly as we do, as soon as possible! :)

But to be more descriptive - the goal is for these teams to have some visibility and control over planning for their workload, and for you to have all the resources you need for a launch. So I think it's helpful to look at it across a couple of horizons:

• Annual - work with product to give people a sense of the biggest launches coming for the year, at the beginning of the year. This should be something you and product leaders can present to the whole company at a kickoff or offsite or all-hands, whatever you normally have. But it's also something you should be able to present to a marketing leadership group so they know the general shape the year will take...this is a great time to try to line up with events or other items on the calendar.
• Biannual / Quarterly - we do our resource planning biannually, but had previously done it quarterly. Whatever you do, use this time to talk more literally about the big things coming during that planning cycle, what resources you think you'll need, what audiences you'll need to target. 
• Kickoff - once you have the specifics of the launch, and know who will be working on it, have your kickoff meeting. We do a messaging source doc and launch plan for each of our launches, but it's ok to have the kick-off before all of this is finalized, you want people to be able to start planning for the workload so you can start with the high level and set expectations for when the detail will be available.

This is super high level because each organization is able to work at different speeds. It's important to understand the work-back schedule of your most important and longest lead-time channels, for us that's often web and/or video. So we know we need to kick off X weeks ahead if we expect to have a brand new tier one product page up, less if it's only a landing page, etc. Develop a relationship with the channel teams so you have regular check-ins and planning cycles where you can balance giving them advanced notice and setting expectations for when you'll have all the details they need. That way they aren't surprised, but also aren't spinning up before you're ready.

Is it ever wrong to start as soon as possible?! Early in the ideation/business case phase, I try to keep it narrow. I'll tap a few members of the GTM team to help me brainstorm use cases and value proposition, which we then refine through additional research and customer validation. It's helpful to have early champions that you can pull in as you are launching -- they can help generate excitement and understanding. 

If it's a really new motion for the company (e.g., the first time you're introducing a new product or a new way of pricing), or a new buyer, you really can't start those conversations with teams like finance, sales, customer success soon enough. Ideally 6+ months. If it's not such a new paradigm, then I'd say on average I aim to start about 3-4 months in advance. We start with a single member of the key teams, and then as we get closer to launch (6 weeks-ish) start to pull in others.

Ultimately, it depends on your organization. How fast it moves, whether it's a consensus-driven culture, etc. At a previous company, I authored a launch playbook, and after every major launch, we would do a retrospective and refine it. I really like to approach launches as something we can continuously improve upon rather than a big unveil or overhaul. 

I hate to answer with "it depends", but it does! Atlassian has ~25 cross-functional teams that I worked with as the launch came closer. Some of those teams required 4 months of lead time to make sure everything was done on time! Others came in during the last few weeks. I found that it was important to cast a wide net early on and to be curious. Your curiosity will uncover more dependencies than trying to think of every possible dependent team if your org is larger.

For smaller companies, I found that most of the work was crammed into the two months prior to the launch. If I needed a new email the day before the launch, this was fairly easy to do myself. At Atlassian, where an email team manages such sends, I had to have content finalized weeks before the launch day. 

A good rule of thumb is that you should double or triple the SLA of the teams you work with. If an email team requires 2 weeks of heads up, try to get engaged with them 4-6 weeks beforehand so that you have a more elevated final result that they feel bought in to.

If I had to do things differently, I would have set up some larger meetings where I explained the launch to at least one member of each team. I was hesitant to have large meetings because, well, large meetings aren't great, but that would've made things go more smoothly.

Good question. Part of this depends on what channels and content you (and the other teams) plan to create. Try to take at least one of the longer-tail pieces of your launch, like potentially a web-based tool and then work backward to determine what the right timeframe is.

For many launches, I typically start the planning conversation with other teams in marketing and throughout the business when the product is in an early alpha. At that time we'll meet occasionally and sync on the product narrative, as well as begin to think through demand generation, adoption, and more. As the product gets closer to beta these plans get finalized and typically a lot of the execution of launch-related content and assets is underway at that time.

All of the above really depends on the scale and importance of the launch as well. For a large launch, all of the above is true while a small scale launch the process will typically start much later around the time of public beta. 

As early as you can! Even before the clickable demo is built, it's imparative to meet with the PDE team to start planning vision PPT decks, use case stories, personas, etc. Then, as soon as the timeline is built out where you know when it will be in pilot phase, you need to sync up with the Sales teams and do sales enablement training. Throughout the entire GTM process, make sure all channels are involved in stakeholder meetings to provide feedback along the way on what is being developed.