It completely depends on your niche, your goals, your competition, the amount of time you have, and the expertise level of the person writing the description.
Your Niche and Competition:
Google the top 5-10 product descriptions from a high traffic, important keyword in your industry. For best results, track at least 10 keywords. Each keyword can be different, so you may have to be careful there. Look at the top 5-10 results for a product-related term. Are they long descriptions? Are they short. Are there none? What's the content of them? What features and topics do they have? How's the UX and mobile? I could go on and on.
Your Goals:
Are you looking to do a 10X product description or just throw a little something together. I always recommend re-writing the manufacturer's information in your own words at the very least. See Rands Why Good Unique Content Needs to Die for more
This brings in the writer's Experience Level
Are you a beginner or an expert in the niche? You may not be able to write 10X content if you know nothing. Try rewriting the manufacturer's info for starters. You will learn a lot. Never copy and paste from another website into your own!
Which ends with time and scalability
Do you have all the time in the world or 5 minutes per description. Most medium difficulty level niches require a store with lots of content and at least 200 products just to get started, but it can vary widely and it really depends. Try balancing between not spending enough time and spending all day on one 10X product. Try making your top 10 hitters (by profit) 10X. or make your top 30 or 50 10X. 80% of your profit comes from 20% of your products. Sometimes it's more extreme.
I hope I have helped.
