I don't see how you determine when to sell because correct me if I am wrong but the higher the value of dash coin the more $ the MNs will receive every month. Ideally the coin reaches $2,000,000 and the MNs are crushing it. So my vote is "Never"
To me it's not a question of "selling it", it's more a question of when I might want to "spend" it. I don't really see myself selling it just to hold onto fiat instead, I would only do that if there was something fairly large I wanted to buy or donate towards that I don't have enough Fiat to do. And that kind of decision is more about me and what I want to do in life, than it is about what the price of Dash is.