In order to properly test your ads and for the algorithm to optimize, you're going to need to invest some money in the Facebook ads platform. But exactly how much should you dole out each day or even each month?
First, it does depend on your budget. If you have a brand new store, you're likely bootstrapping and have to invest your own cash into testing your ads. More often than not, your funds will be limited in the beginning.
The minimum required ad spend in Facebook is pretty low per ad set; technically you can go as low as $1-$5 per day. But should you actually go this low? Not necessarily. That amounts to only $150 per month and will not give you much data, nor will it allow Facebook's algorithm to optimize.
In a perfect world, you should invest at least $2000 per month in your Facebook ads when your site is getting sales. If you don't have sales yet and your conversion rate is low (under 2%), start small with around $500-$1000 per month in ad spend while you work on your website conversion rate. This will help give you site data while working on improving your conversion rate. Once your website metrics start improving and you start getting sales, that would be a good time to increase your ad spend to above $2000 per month and ramp up your ad testing.