What's your best advice with how to respond to a customer no-decision? As in, "let's revisit this in 6 months."

When a prospect responds with this type of deflection, it's a sign of having a weak position in the sales process. Meaning, the prospect "pain" doesn't hurt enough to require immediate resolution, or the solution position isn't properly aligned to solving the prospect's pain. A good response to the "let's revisit this in 6 months" response is to walk the opportunity back with the prospect. For example: "Thank you for your feedback. It sounds like things may have changed. When we initially began our discussions you stated that you [describe the prospect identified pain] which needed to be addressed by [insert the prospect's timeline]. What has changed?"

You're going to learn where the opportunity went sideways. This triage technique will help you understand where either the deal wasn't properly qualified, or properly aligned to prospect need. In many cases, we learn that the sales rep created the prospect pain and timeline, then tried to artificially create motivation. It sounds harsh, but when reviewing with the sales rep, it's fair to ask, "Was this the prospect's defined pain and timeline, or did you define their pain and timeline?"

Use it as an opportunity to learn what will be different then and how you can use the time to prepare a solution that will improve their business. Call once you’ve got it and let them decide how happy they want to be and when.

If you have built a solid relationship with the customer I think you can ask them what they hope to gain by waiting 6 months.  By asking this and getting a straight forward answer you should be able to either A, move it forward by over coming their concerns or B, realizing that this hurdle can not be overcome and set up time to revisit after said period of time as expired.  

Do mean a negative decision, Like No or that the Customer takes no decision weather yes nor no?

This is not as uncommon as one might think, I encounter it even still and when leading sales teams encountered it with well experienced sales professionals.

My go to question when encountering this and to focus the customer on describing the urgency to solve and the benefits of solving now versus 6 months is to ask the customer “What are you putting at risk by waiting an additional six months to kick-off your resolution plan for [insert need your trying to validate]?”

This is because in my experience, whether you encounter this in the beginning, or late, stages of an opportunity should clearly demonstrate to the representative (and the sales leader) that we failed at validating the customer’s critical issue needing to be solved, quantifying the value and impact the customer would realize by resolving the issue, the unique value our solution offers the customer to help to resolve it, and lastly our differentiated credibility to guide them in doing so.