How far into the future do you believe Product Marketers need visibility into the product roadmap to be effective and why?

This will depend on the type of business you are in. Hardware? Enterprise? These have longer development and sales cycles so the teams have to plan much further ahead vs. a self service model where products tend to ship faster. Also the maturity of the company will make a difference because Product teams at younger companies tend to have shorter foresight themselves because they are potentially still experimenting with product market fit.

Taking into account the context of your world, apply a tiering system to the roadmap and estimate how long it takes for you to flawlessly execute a T1 launch - including localization if you are a global company. At a bare minimum, you need that much time. However ideally, you are meeting with your Product team each planning cycle to go over their plans, make your contributions and identify and align on key moments where you’ll really go big on marketing.

Agree with Mike! PMM needs to be part of the roadmap development process. Most companies do annual planning. So you should know where you are taking the product to in the next year. Then, what are the quarterly feature release roadmaps.

I would highly recommend having visibility and clarity on your product vision and mission as part of undersetanding the roadmap. To me, the product roadmap should be the path to fulfill this larger mission and objective. 

There is a 20-year-roadmap exercise in Jake Knapp's Brand Design sprint that I found really helpful for understanding long term objectives and positioning for the brand (and ultimately the product for my company): https://library.gv.com/the-three-hour-brand-sprint-3ccabf4b768a

I agree with Mike. Being part of the product life-cycle planning gives you visibility into the future as well as whats currently ready to ship. I prefer to create hierarchy of messaging - Company, portfolio (if it exist), individual product, so that some of the broader messaging can stay for 12-18 months while actual product gets built out to achieve that broader vision we set forth together as a team. It also gives marketing enough to provide air cover for sales. Also, the higher level messaging can really help in opening doors with CXO level audience. Again this is very much true in large enterprise to mid-market type enterprise deals, which require strategic selling and building a channel ecosystem. May not be as needed for consumer. 

 As a PMM, you should have visibility into the entire roadmap - as far out as it goes. You and your PM should basically be joined at the hip, and you should be an active collaborator in influencing and developing the roadmap. Being a part of that process will give you deeper insight into the problems you are solving, and will let you write better messaging. 

Even if that weren't all true, though, what possible downside is there in not seeing the whole thing? I guess you could make some convoluted argument that the PMM may get tempted to get too far over her skis and start messaging to futures that don't exist? I'd counter that a good PMM should be able to resist that temptation and walk that fine line. 

I will be the devil's advocate. The PMM needs visibility into the roadmap as far as it relates to current products and next products. I break the product world into 3 basic categories: current products, next products, and future products. Focusing attention on features and products that are too far off in the distance can be a distraction (future products). Of course you'll want to know what is expected to be launched (next products) so you can plan for them. Your question included the phrase "to be effective". To that end I recommend focusing on current products and next products to drive revenue and adoption.