Timeliness is a common concern of users running their first sale. Product Discount sends a series of updates to Shopify ("API calls") to change the price, and add/remove product tags. Shopify places an "API call limit" on how many updates can be done in a period. Depending on the number of products, the time for the offer to completely update will vary from a few minutes for a collection or several hours for a store-wide sale. A sale that starts at midnight means that sale prices will start to be added one-by-one; the prices are not all updated at midnight. Consider this when scheduling a sale to start or end at a given time.
Note: A general rule of thumb we use is to allow 1 hour per 1000 variants being discounted.
A common mistake with creating the first discount is the "Select All Results" button. Selecting this, without filtering the list, will apply the discount to the entire store. For some stores, this can include thousands of products.
Select from the drop-down list, and then select Search. The list of products from that collection/vendor/product type will appear on the left. From here, select the "Select All Results" to include those products.