1) Make sure your CTR (Click Through Rate) is 4% and above (with good sales rate). If you choose CPC, a lot of people will click your post and more people will buy (until your CTR reached 4% and above). Once this is reached, it is advisable to change from CPC to CPM so that your post will be advertised speedily with much lower cost.
2) Check the effectiveness of the FB Ads on daily basis.
If your FB Ads runs and generate response or sales, let the advert runs. If there is no response, just stop the FB Ads and create a new one.