Shopify will invoice you every 30 days. Fees will appear on an invoice, which will be sent to the Store Owner via the email provided. As well, an invoice will appear in Settings/Billing. You can also view pending/upcoming charges in the Settings/Billing area as well. If you’re not receiving these emails, then I would check to ensure the email in Settings/General is correct. If it is, and the email is working, then these emails may be landing in your spam/junk folder.
If not, then I would recommend contacting Shopify Support to see if these emails are being sent to the email we have on file. You can contact support by logging into your store through the help center, searching your question/issue, then selecting “continue” at the bottom of the page that displays the search results.