Write product descriptions around accurate buying questions. A description's length and keyword count are writing inputs, not evidence that it will convert.
The 150/850 framework is a drafting heuristic: make roughly the opening 150 characters identify the product and its main use, then use the remaining explanatory space for relevant attributes, fit and limitations. It is not a promise that a fixed number of characters will appear on every Shopping surface.
For an illustrative pair of travel headphones, the opening might read:
Wireless noise-cancelling headphones with a 30-hour battery for travel, work and commuting.
That sentence is suitable only if the actual product supports those claims. Follow it with the details a buyer needs: compatibility, comfort, charging, what is included and what noise cancellation can and cannot do. Product specifications remain useful when they answer those questions.
Do not force eight to twelve keywords into every description. Use natural language that matches the product, avoid repetition and retain consistency with the landing page. Google's current product specification controls the supported field and policy requirements.
The former average-length and CTR/CPC/conversion benchmarks did not establish that longer descriptions caused better results. A useful audit reads the description beside the product page, checks factual gaps and tests a bounded revision with comparable exposure.
Take the idea further
Open the description framework