For sales cycles that are "long" 18 months+ what KPIs do you recommend product marketers use to determine ROI / impact / influence on the deal. Industry context: enterprise healthcare IT software

That's would be a tough problem if you are trying to understand the impact on one deal over a period of 18 months. I would flip this on its head and instead try to focus on measuring pipeline progression and map it to product marketing work. 

For example:

Let's say your sales funnel is a 6 stage funnel. Start by understanding the current health of your funnel. Analyze data from the last 3-4 years to see where the which stage do the opportunities are stuck for the longest. For example, let's say that the deals are stuck in stage 4. Now, this could just be the nature of your industry but most likely there would be some things thatmight help you accelerate this stage by 2-3 weeks on average. Multiply that with the number of deals in that stage, and you will see a huge impact on the "Deal Cycle" metric. To make it more real, let's say currently stage 4 is 100 days average. You realize that by creating an ROI report and a set of relevant customer case studies, your buyers would make the decision in 75 days. If you have 1000 deals in stage 4, you have basically reduced the sales cycle by 25 days on average and 25000 days in total for your existing funnel. That's a lot! :) 

Some metrics that would help you assess the health of your pipeline are =

Average Sales Cycle
Last Activity Date on opportunities
Push Count
Average Deal Size
Slice & dice by Customer Industry & Win Rate - move the needle in a few of these
Regional Distribution - again pick regions where you can make a significant impact
Funnel Shape: Better when you have a funnel that looks like a funnel. Many times, one of the stages would have a ton of pipeline stuck. The funnel then is distorted. 

Naturally you want to look at how your content and its assets have touched the sales cycle. Typically here are a few ways I tend to look at metrics with long sales cycles (upmarket):

• Conversions NARR
• Attachment rates - new logo and cross selling
• Product ASP - the sales price, keep a close eye on this
• Industry/Segment NARR
• Gross Pipeline
• Win/close rates