Dropshipping Facebook ads

Hello
Considering I found a product for dropshipping and I've made the right targeting. For how long should I run the ad to know if it's good product or not.
Usually and before running the ad i estimate a CPA amount and if I passed the amount without selling anything I stop the ad.
Please let me know your opinions.

No. Stop trying to send cold traffic to a "winner". Especially if you have to ask a question like that. What is this, 2015? Are 'gurus' still trying to sell this as a viable business plan???

This question gets asked a lot and Im still confused about it myself.

For example: If after cost, a product yields me $10 in profit. If for every $50 in FB ads I spend I only convert 2 sales, its foolish to keep running the same ads. Right?

However, so called "gurus" may tell you to spend $50 in ads to try out a new product. Wouldnt it make sense to just spend $10 in Facebook ads instead?